Merge branch 'bc/object-id'

Conversion from uchar[20] to struct object_id continues.

* bc/object-id: (53 commits)
  object: convert parse_object* to take struct object_id
  tree: convert parse_tree_indirect to struct object_id
  sequencer: convert do_recursive_merge to struct object_id
  diff-lib: convert do_diff_cache to struct object_id
  builtin/ls-tree: convert to struct object_id
  merge: convert checkout_fast_forward to struct object_id
  sequencer: convert fast_forward_to to struct object_id
  builtin/ls-files: convert overlay_tree_on_cache to object_id
  builtin/read-tree: convert to struct object_id
  sha1_name: convert internals of peel_onion to object_id
  upload-pack: convert remaining parse_object callers to object_id
  revision: convert remaining parse_object callers to object_id
  revision: rename add_pending_sha1 to add_pending_oid
  http-push: convert process_ls_object and descendants to object_id
  refs/files-backend: convert many internals to struct object_id
  refs: convert struct ref_update to use struct object_id
  ref-filter: convert some static functions to struct object_id
  Convert struct ref_array_item to struct object_id
  Convert the verify_pack callback to struct object_id
  Convert lookup_tag to struct object_id
  ...

106 files changed, 1173 insertions, 1134 deletions

diff --git a/archive.c b/archive.c
index 60b8891986..b15a922dab 100644
--- a/archive.c
+++ b/archive.c
@@ -360,7 +360,7 @@ static void parse_treeish_arg(const char **argv,
 	if (get_sha1(name, oid.hash))
 		die("Not a valid object name");
 
-	commit = lookup_commit_reference_gently(oid.hash, 1);
+	commit = lookup_commit_reference_gently(&oid, 1);
 	if (commit) {
 		commit_sha1 = commit->object.oid.hash;
 		archive_time = commit->date;
@@ -369,7 +369,7 @@ static void parse_treeish_arg(const char **argv,
 		archive_time = time(NULL);
 	}
 
-	tree = parse_tree_indirect(oid.hash);
+	tree = parse_tree_indirect(&oid);
 	if (tree == NULL)
 		die("not a tree object");
 
@@ -383,7 +383,7 @@ static void parse_treeish_arg(const char **argv,
 		if (err || !S_ISDIR(mode))
 			die("current working directory is untracked");
 
-		tree = parse_tree_indirect(tree_oid.hash);
+		tree = parse_tree_indirect(&tree_oid);
 	}
 	ar_args->tree = tree;
 	ar_args->commit_sha1 = commit_sha1;
diff --git a/bisect.c b/bisect.c
index 08c9fb7266..bb5af3ea3d 100644
--- a/bisect.c
+++ b/bisect.c
@@ -705,7 +705,7 @@ static int bisect_checkout(const unsigned char *bisect_rev, int no_checkout)
 
 static struct commit *get_commit_reference(const struct object_id *oid)
 {
-	struct commit *r = lookup_commit_reference(oid->hash);
+	struct commit *r = lookup_commit_reference(oid);
 	if (!r)
 		die(_("Not a valid commit name %s"), oid_to_hex(oid));
 	return r;
diff --git a/blob.c b/blob.c
index 1fcb8e44b0..fa2ab4f7a7 100644
--- a/blob.c
+++ b/blob.c
@@ -3,11 +3,11 @@
 
 const char *blob_type = "blob";
 
-struct blob *lookup_blob(const unsigned char *sha1)
+struct blob *lookup_blob(const struct object_id *oid)
 {
-	struct object *obj = lookup_object(sha1);
+	struct object *obj = lookup_object(oid->hash);
 	if (!obj)
-		return create_object(sha1, alloc_blob_node());
+		return create_object(oid->hash, alloc_blob_node());
 	return object_as_type(obj, OBJ_BLOB, 0);
 }
 
diff --git a/blob.h b/blob.h
index 59b394eea3..4460616831 100644
--- a/blob.h
+++ b/blob.h
@@ -9,7 +9,7 @@ struct blob {
 	struct object object;
 };
 
-struct blob *lookup_blob(const unsigned char *sha1);
+struct blob *lookup_blob(const struct object_id *oid);
 
 int parse_blob_buffer(struct blob *item, void *buffer, unsigned long size);
 
diff --git a/branch.c b/branch.c
index bb9eb60dcd..985316eb76 100644
--- a/branch.c
+++ b/branch.c
@@ -191,9 +191,9 @@ int validate_new_branchname(const char *name, struct strbuf *ref,
 
 	if (!attr_only) {
 		const char *head;
-		unsigned char sha1[20];
+		struct object_id oid;
 
-		head = resolve_ref_unsafe("HEAD", 0, sha1, NULL);
+		head = resolve_ref_unsafe("HEAD", 0, oid.hash, NULL);
 		if (!is_bare_repository() && head && !strcmp(head, ref->buf))
 			die(_("Cannot force update the current branch."));
 	}
@@ -233,7 +233,7 @@ void create_branch(const char *name, const char *start_name,
 		   int quiet, enum branch_track track)
 {
 	struct commit *commit;
-	unsigned char sha1[20];
+	struct object_id oid;
 	char *real_ref;
 	struct strbuf ref = STRBUF_INIT;
 	int forcing = 0;
@@ -253,7 +253,7 @@ void create_branch(const char *name, const char *start_name,
 	}
 
 	real_ref = NULL;
-	if (get_sha1(start_name, sha1)) {
+	if (get_oid(start_name, &oid)) {
 		if (explicit_tracking) {
 			if (advice_set_upstream_failure) {
 				error(_(upstream_missing), start_name);
@@ -265,7 +265,7 @@ void create_branch(const char *name, const char *start_name,
 		die(_("Not a valid object name: '%s'."), start_name);
 	}
 
-	switch (dwim_ref(start_name, strlen(start_name), sha1, &real_ref)) {
+	switch (dwim_ref(start_name, strlen(start_name), oid.hash, &real_ref)) {
 	case 0:
 		/* Not branching from any existing branch */
 		if (explicit_tracking)
@@ -286,9 +286,9 @@ void create_branch(const char *name, const char *start_name,
 		break;
 	}
 
-	if ((commit = lookup_commit_reference(sha1)) == NULL)
+	if ((commit = lookup_commit_reference(&oid)) == NULL)
 		die(_("Not a valid branch point: '%s'."), start_name);
-	hashcpy(sha1, commit->object.oid.hash);
+	oidcpy(&oid, &commit->object.oid);
 
 	if (reflog)
 		log_all_ref_updates = LOG_REFS_NORMAL;
@@ -306,7 +306,7 @@ void create_branch(const char *name, const char *start_name,
 		transaction = ref_transaction_begin(&err);
 		if (!transaction ||
 		    ref_transaction_update(transaction, ref.buf,
-					   sha1, forcing ? NULL : null_sha1,
+					   oid.hash, forcing ? NULL : null_sha1,
 					   0, msg, &err) ||
 		    ref_transaction_commit(transaction, &err))
 			die("%s", err.buf);
diff --git a/builtin/am.c b/builtin/am.c
index 8e9ac1144d..2ec1d7b71e 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -1145,7 +1145,7 @@ static int index_has_changes(struct strbuf *sb)
 		DIFF_OPT_SET(&opt, EXIT_WITH_STATUS);
 		if (!sb)
 			DIFF_OPT_SET(&opt, QUICK);
-		do_diff_cache(head.hash, &opt);
+		do_diff_cache(&head, &opt);
 		diffcore_std(&opt);
 		for (i = 0; sb && i < diff_queued_diff.nr; i++) {
 			if (i)
@@ -1447,9 +1447,9 @@ static void write_index_patch(const struct am_state *state)
 	FILE *fp;
 
 	if (!get_sha1_tree("HEAD", head.hash))
-		tree = lookup_tree(head.hash);
+		tree = lookup_tree(&head);
 	else
-		tree = lookup_tree(EMPTY_TREE_SHA1_BIN);
+		tree = lookup_tree(&empty_tree_oid);
 
 	fp = xfopen(am_path(state, "patch"), "w");
 	init_revisions(&rev_info, NULL);
@@ -1482,7 +1482,7 @@ static int parse_mail_rebase(struct am_state *state, const char *mail)
 	if (get_mail_commit_oid(&commit_oid, mail) < 0)
 		die(_("could not parse %s"), mail);
 
-	commit = lookup_commit_or_die(commit_oid.hash, mail);
+	commit = lookup_commit_or_die(&commit_oid, mail);
 
 	get_commit_info(state, commit);
 
@@ -1612,7 +1612,7 @@ static int fall_back_threeway(const struct am_state *state, const char *index_pa
 		init_revisions(&rev_info, NULL);
 		rev_info.diffopt.output_format = DIFF_FORMAT_NAME_STATUS;
 		diff_opt_parse(&rev_info.diffopt, &diff_filter_str, 1, rev_info.prefix);
-		add_pending_sha1(&rev_info, "HEAD", our_tree.hash, 0);
+		add_pending_oid(&rev_info, "HEAD", &our_tree, 0);
 		diff_setup_done(&rev_info.diffopt);
 		run_diff_index(&rev_info, 1);
 	}
@@ -1677,7 +1677,7 @@ static void do_commit(const struct am_state *state)
 
 	if (!get_sha1_commit("HEAD", parent.hash)) {
 		old_oid = &parent;
-		commit_list_insert(lookup_commit(parent.hash), &parents);
+		commit_list_insert(lookup_commit(&parent), &parents);
 	} else {
 		old_oid = NULL;
 		say(state, stderr, _("applying to an empty history"));
@@ -2039,11 +2039,11 @@ static int clean_index(const struct object_id *head, const struct object_id *rem
 	struct tree *head_tree, *remote_tree, *index_tree;
 	struct object_id index;
 
-	head_tree = parse_tree_indirect(head->hash);
+	head_tree = parse_tree_indirect(head);
 	if (!head_tree)
 		return error(_("Could not parse object '%s'."), oid_to_hex(head));
 
-	remote_tree = parse_tree_indirect(remote->hash);
+	remote_tree = parse_tree_indirect(remote);
 	if (!remote_tree)
 		return error(_("Could not parse object '%s'."), oid_to_hex(remote));
 
@@ -2055,7 +2055,7 @@ static int clean_index(const struct object_id *head, const struct object_id *rem
 	if (write_cache_as_tree(index.hash, 0, NULL))
 		return -1;
 
-	index_tree = parse_tree_indirect(index.hash);
+	index_tree = parse_tree_indirect(&index);
 	if (!index_tree)
 		return error(_("Could not parse object '%s'."), oid_to_hex(&index));
 
diff --git a/builtin/blame.c b/builtin/blame.c
index f00eda1637..1043e5376f 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -563,7 +563,7 @@ static struct origin *find_origin(struct scoreboard *sb,
 	diff_setup_done(&diff_opts);
 
 	if (is_null_oid(&origin->commit->object.oid))
-		do_diff_cache(parent->tree->object.oid.hash, &diff_opts);
+		do_diff_cache(&parent->tree->object.oid, &diff_opts);
 	else
 		diff_tree_sha1(parent->tree->object.oid.hash,
 			       origin->commit->tree->object.oid.hash,
@@ -633,7 +633,7 @@ static struct origin *find_rename(struct scoreboard *sb,
 	diff_setup_done(&diff_opts);
 
 	if (is_null_oid(&origin->commit->object.oid))
-		do_diff_cache(parent->tree->object.oid.hash, &diff_opts);
+		do_diff_cache(&parent->tree->object.oid, &diff_opts);
 	else
 		diff_tree_sha1(parent->tree->object.oid.hash,
 			       origin->commit->tree->object.oid.hash,
@@ -1272,7 +1272,7 @@ static void find_copy_in_parent(struct scoreboard *sb,
 		DIFF_OPT_SET(&diff_opts, FIND_COPIES_HARDER);
 
 	if (is_null_oid(&target->commit->object.oid))
-		do_diff_cache(parent->tree->object.oid.hash, &diff_opts);
+		do_diff_cache(&parent->tree->object.oid, &diff_opts);
 	else
 		diff_tree_sha1(parent->tree->object.oid.hash,
 			       target->commit->tree->object.oid.hash,
@@ -2253,7 +2253,7 @@ static struct commit_list **append_parent(struct commit_list **tail, const struc
 {
 	struct commit *parent;
 
-	parent = lookup_commit_reference(oid->hash);
+	parent = lookup_commit_reference(oid);
 	if (!parent)
 		die("no such commit %s", oid_to_hex(oid));
 	return &commit_list_insert(parent, tail)->next;
@@ -2461,7 +2461,7 @@ static const char *dwim_reverse_initial(struct scoreboard *sb)
 	 */
 	struct object *obj;
 	struct commit *head_commit;
-	unsigned char head_sha1[20];
+	struct object_id head_oid;
 
 	if (sb->revs->pending.nr != 1)
 		return NULL;
@@ -2473,9 +2473,9 @@ static const char *dwim_reverse_initial(struct scoreboard *sb)
 		return NULL;
 
 	/* Do we have HEAD? */
-	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, head_sha1, NULL))
+	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, head_oid.hash, NULL))
 		return NULL;
-	head_commit = lookup_commit_reference_gently(head_sha1, 1);
+	head_commit = lookup_commit_reference_gently(&head_oid, 1);
 	if (!head_commit)
 		return NULL;
 
diff --git a/builtin/branch.c b/builtin/branch.c
index 48a513a84d..83fcda43dc 100644
--- a/builtin/branch.c
+++ b/builtin/branch.c
@@ -124,7 +124,7 @@ static int branch_merged(int kind, const char *name,
 		    (reference_name = reference_name_to_free =
 		     resolve_refdup(upstream, RESOLVE_REF_READING,
 				    oid.hash, NULL)) != NULL)
-			reference_rev = lookup_commit_reference(oid.hash);
+			reference_rev = lookup_commit_reference(&oid);
 	}
 	if (!reference_rev)
 		reference_rev = head_rev;
@@ -157,7 +157,7 @@ static int check_branch_commit(const char *branchname, const char *refname,
 			       const struct object_id *oid, struct commit *head_rev,
 			       int kinds, int force)
 {
-	struct commit *rev = lookup_commit_reference(oid->hash);
+	struct commit *rev = lookup_commit_reference(oid);
 	if (!rev) {
 		error(_("Couldn't look up commit object for '%s'"), refname);
 		return -1;
@@ -211,7 +211,7 @@ static int delete_branches(int argc, const char **argv, int force, int kinds,
 	}
 
 	if (!force) {
-		head_rev = lookup_commit_reference(head_oid.hash);
+		head_rev = lookup_commit_reference(&head_oid);
 		if (!head_rev)
 			die(_("Couldn't look up commit object for HEAD"));
 	}
diff --git a/builtin/checkout.c b/builtin/checkout.c
index 6c3d2e4f4c..65877bacb1 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -393,7 +393,7 @@ static int checkout_paths(const struct checkout_opts *opts,
 		die(_("unable to write new index file"));
 
 	read_ref_full("HEAD", 0, rev.hash, NULL);
-	head = lookup_commit_reference_gently(rev.hash, 1);
+	head = lookup_commit_reference_gently(&rev, 1);
 
 	errs |= post_checkout_hook(head, head, 0);
 	return errs;
@@ -527,10 +527,10 @@ static int merge_working_tree(const struct checkout_opts *opts,
 			setup_standard_excludes(topts.dir);
 		}
 		tree = parse_tree_indirect(old->commit ?
-					   old->commit->object.oid.hash :
-					   EMPTY_TREE_SHA1_BIN);
+					   &old->commit->object.oid :
+					   &empty_tree_oid);
 		init_tree_desc(&trees[0], tree->buffer, tree->size);
-		tree = parse_tree_indirect(new->commit->object.oid.hash);
+		tree = parse_tree_indirect(&new->commit->object.oid);
 		init_tree_desc(&trees[1], tree->buffer, tree->size);
 
 		ret = unpack_trees(2, trees, &topts);
@@ -721,7 +721,7 @@ static int add_pending_uninteresting_ref(const char *refname,
 					 const struct object_id *oid,
 					 int flags, void *cb_data)
 {
-	add_pending_sha1(cb_data, refname, oid->hash, UNINTERESTING);
+	add_pending_oid(cb_data, refname, oid, UNINTERESTING);
 	return 0;
 }
 
@@ -807,7 +807,7 @@ static void orphaned_commit_warning(struct commit *old, struct commit *new)
 	add_pending_object(&revs, object, oid_to_hex(&object->oid));
 
 	for_each_ref(add_pending_uninteresting_ref, &revs);
-	add_pending_sha1(&revs, "HEAD", new->object.oid.hash, UNINTERESTING);
+	add_pending_oid(&revs, "HEAD", &new->object.oid, UNINTERESTING);
 
 	refs = revs.pending;
 	revs.leak_pending = 1;
@@ -834,7 +834,7 @@ static int switch_branches(const struct checkout_opts *opts,
 	memset(&old, 0, sizeof(old));
 	old.path = path_to_free = resolve_refdup("HEAD", 0, rev.hash, &flag);
 	if (old.path)
-		old.commit = lookup_commit_reference_gently(rev.hash, 1);
+		old.commit = lookup_commit_reference_gently(&rev, 1);
 	if (!(flag & REF_ISSYMREF))
 		old.path = NULL;
 
@@ -1048,10 +1048,10 @@ static int parse_branchname_arg(int argc, const char **argv,
 	else
 		new->path = NULL; /* not an existing branch */
 
-	new->commit = lookup_commit_reference_gently(rev->hash, 1);
+	new->commit = lookup_commit_reference_gently(rev, 1);
 	if (!new->commit) {
 		/* not a commit */
-		*source_tree = parse_tree_indirect(rev->hash);
+		*source_tree = parse_tree_indirect(rev);
 	} else {
 		parse_commit_or_die(new->commit);
 		*source_tree = new->commit->tree;
diff --git a/builtin/clone.c b/builtin/clone.c
index afab299433..743f16ae2a 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -685,7 +685,7 @@ static void update_head(const struct ref *our, const struct ref *remote,
 			install_branch_config(0, head, option_origin, our->name);
 		}
 	} else if (our) {
-		struct commit *c = lookup_commit_reference(our->old_oid.hash);
+		struct commit *c = lookup_commit_reference(&our->old_oid);
 		/* --branch specifies a non-branch (i.e. tags), detach HEAD */
 		update_ref(msg, "HEAD", c->object.oid.hash,
 			   NULL, REF_NODEREF, UPDATE_REFS_DIE_ON_ERR);
@@ -742,7 +742,7 @@ static int checkout(int submodule_progress)
 	opts.src_index = &the_index;
 	opts.dst_index = &the_index;
 
-	tree = parse_tree_indirect(oid.hash);
+	tree = parse_tree_indirect(&oid);
 	parse_tree(tree);
 	init_tree_desc(&t, tree->buffer, tree->size);
 	if (unpack_trees(1, &t, &opts) < 0)
diff --git a/builtin/commit-tree.c b/builtin/commit-tree.c
index 605017261c..f39c2b2737 100644
--- a/builtin/commit-tree.c
+++ b/builtin/commit-tree.c
@@ -58,7 +58,7 @@ int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 			if (get_sha1_commit(argv[i], oid.hash))
 				die("Not a valid object name %s", argv[i]);
 			assert_sha1_type(oid.hash, OBJ_COMMIT);
-			new_parent(lookup_commit(oid.hash), &parents);
+			new_parent(lookup_commit(&oid), &parents);
 			continue;
 		}
 
diff --git a/builtin/commit.c b/builtin/commit.c
index 9028bfacf8..1d191a49f8 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -313,7 +313,7 @@ static void create_base_index(const struct commit *current_head)
 	opts.dst_index = &the_index;
 
 	opts.fn = oneway_merge;
-	tree = parse_tree_indirect(current_head->object.oid.hash);
+	tree = parse_tree_indirect(&current_head->object.oid);
 	if (!tree)
 		die(_("failed to unpack HEAD tree object"));
 	parse_tree(tree);
@@ -1434,7 +1434,7 @@ static void print_summary(const char *prefix, const struct object_id *oid,
 	struct strbuf author_ident = STRBUF_INIT;
 	struct strbuf committer_ident = STRBUF_INIT;
 
-	commit = lookup_commit(oid->hash);
+	commit = lookup_commit(oid);
 	if (!commit)
 		die(_("couldn't look up newly created commit"));
 	if (parse_commit(commit))
@@ -1658,7 +1658,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	if (get_sha1("HEAD", oid.hash))
 		current_head = NULL;
 	else {
-		current_head = lookup_commit_or_die(oid.hash, "HEAD");
+		current_head = lookup_commit_or_die(&oid, "HEAD");
 		if (parse_commit(current_head))
 			die(_("could not parse HEAD commit"));
 	}
@@ -1762,7 +1762,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		append_merge_tag_headers(parents, &tail);
 	}
 
-	if (commit_tree_extended(sb.buf, sb.len, active_cache_tree->sha1,
+	if (commit_tree_extended(sb.buf, sb.len, active_cache_tree->oid.hash,
 			 parents, oid.hash, author_ident.buf, sign_commit, extra)) {
 		rollback_index_files();
 		die(_("failed to write commit object"));
diff --git a/builtin/describe.c b/builtin/describe.c
index a5cd8c513f..893c8789f4 100644
--- a/builtin/describe.c
+++ b/builtin/describe.c
@@ -79,13 +79,13 @@ static int replace_name(struct commit_name *e,
 		struct tag *t;
 
 		if (!e->tag) {
-			t = lookup_tag(e->oid.hash);
+			t = lookup_tag(&e->oid);
 			if (!t || parse_tag(t))
 				return 1;
 			e->tag = t;
 		}
 
-		t = lookup_tag(oid->hash);
+		t = lookup_tag(oid);
 		if (!t || parse_tag(t))
 			return 0;
 		*tag = t;
@@ -245,7 +245,7 @@ static unsigned long finish_depth_computation(
 static void display_name(struct commit_name *n)
 {
 	if (n->prio == 2 && !n->tag) {
-		n->tag = lookup_tag(n->oid.hash);
+		n->tag = lookup_tag(&n->oid);
 		if (!n->tag || parse_tag(n->tag))
 			die(_("annotated tag %s not available"), n->path);
 	}
@@ -281,7 +281,7 @@ static void describe(const char *arg, int last_one)
 
 	if (get_oid(arg, &oid))
 		die(_("Not a valid object name %s"), arg);
-	cmit = lookup_commit_reference(oid.hash);
+	cmit = lookup_commit_reference(&oid);
 	if (!cmit)
 		die(_("%s is not a valid '%s' object"), arg, commit_type);
 
@@ -309,7 +309,7 @@ static void describe(const char *arg, int last_one)
 		struct commit *c;
 		struct commit_name *n = hashmap_iter_first(&names, &iter);
 		for (; n; n = hashmap_iter_next(&iter)) {
-			c = lookup_commit_reference_gently(n->peeled.hash, 1);
+			c = lookup_commit_reference_gently(&n->peeled, 1);
 			if (c)
 				c->util = n;
 		}
diff --git a/builtin/diff-tree.c b/builtin/diff-tree.c
index 326f88b657..5ea1c14317 100644
--- a/builtin/diff-tree.c
+++ b/builtin/diff-tree.c
@@ -9,7 +9,7 @@ static struct rev_info log_tree_opt;
 
 static int diff_tree_commit_sha1(const struct object_id *oid)
 {
-	struct commit *commit = lookup_commit_reference(oid->hash);
+	struct commit *commit = lookup_commit_reference(oid);
 	if (!commit)
 		return -1;
 	return log_tree_commit(&log_tree_opt, commit);
@@ -23,7 +23,7 @@ static int stdin_diff_commit(struct commit *commit, const char *p)
 
 	/* Graft the fake parents locally to the commit */
 	while (isspace(*p++) && !parse_oid_hex(p, &oid, &p)) {
-		struct commit *parent = lookup_commit(oid.hash);
+		struct commit *parent = lookup_commit(&oid);
 		if (!pptr) {
 			/* Free the real parent list */
 			free_commit_list(commit->parents);
@@ -44,7 +44,7 @@ static int stdin_diff_trees(struct tree *tree1, const char *p)
 	struct tree *tree2;
 	if (!isspace(*p++) || parse_oid_hex(p, &oid, &p) || *p)
 		return error("Need exactly two trees, separated by a space");
-	tree2 = lookup_tree(oid.hash);
+	tree2 = lookup_tree(&oid);
 	if (!tree2 || parse_tree(tree2))
 		return -1;
 	printf("%s %s\n", oid_to_hex(&tree1->object.oid),
@@ -67,7 +67,7 @@ static int diff_tree_stdin(char *line)
 	line[len-1] = 0;
 	if (parse_oid_hex(line, &oid, &p))
 		return -1;
-	obj = parse_object(oid.hash);
+	obj = parse_object(&oid);
 	if (!obj)
 		return -1;
 	if (obj->type == OBJ_COMMIT)
diff --git a/builtin/diff.c b/builtin/diff.c
index d184aafab9..8c03ddaf58 100644
--- a/builtin/diff.c
+++ b/builtin/diff.c
@@ -381,7 +381,7 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 				add_head_to_pending(&rev);
 				if (!rev.pending.nr) {
 					struct tree *tree;
-					tree = lookup_tree(EMPTY_TREE_SHA1_BIN);
+					tree = lookup_tree(&empty_tree_oid);
 					add_pending_object(&rev, &tree->object, "HEAD");
 				}
 				break;
@@ -395,7 +395,7 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 		const char *name = entry->name;
 		int flags = (obj->flags & UNINTERESTING);
 		if (!obj->parsed)
-			obj = parse_object(obj->oid.hash);
+			obj = parse_object(&obj->oid);
 		obj = deref_tag(obj, NULL, 0);
 		if (!obj)
 			die(_("invalid object '%s' given."), name);
@@ -408,7 +408,7 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 		} else if (obj->type == OBJ_BLOB) {
 			if (2 <= blobs)
 				die(_("more than two blobs given: '%s'"), name);
-			hashcpy(blob[blobs].oid.hash, obj->oid.hash);
+			oidcpy(&blob[blobs].oid, &obj->oid);
 			blob[blobs].name = name;
 			blob[blobs].mode = entry->mode;
 			blobs++;
diff --git a/builtin/fast-export.c b/builtin/fast-export.c
index e0220630d0..969550531b 100644
--- a/builtin/fast-export.c
+++ b/builtin/fast-export.c
@@ -232,7 +232,7 @@ static void export_blob(const struct object_id *oid)
 
 	if (anonymize) {
 		buf = anonymize_blob(&size);
-		object = (struct object *)lookup_blob(oid->hash);
+		object = (struct object *)lookup_blob(oid);
 		eaten = 0;
 	} else {
 		buf = read_sha1_file(oid->hash, &type, &size);
@@ -240,7 +240,7 @@ static void export_blob(const struct object_id *oid)
 			die ("Could not read blob %s", oid_to_hex(oid));
 		if (check_sha1_signature(oid->hash, buf, size, typename(type)) < 0)
 			die("sha1 mismatch in blob %s", oid_to_hex(oid));
-		object = parse_object_buffer(oid->hash, type, size, buf, &eaten);
+		object = parse_object_buffer(oid, type, size, buf, &eaten);
 	}
 
 	if (!object)
@@ -777,7 +777,7 @@ static struct commit *get_commit(struct rev_cmdline_entry *e, char *full_name)
 
 		/* handle nested tags */
 		while (tag && tag->object.type == OBJ_TAG) {
-			parse_object(tag->object.oid.hash);
+			parse_object(&tag->object.oid);
 			string_list_append(&extra_refs, full_name)->util = tag;
 			tag = (struct tag *)tag->tagged;
 		}
@@ -938,7 +938,7 @@ static void import_marks(char *input_file)
 			/* only commits */
 			continue;
 
-		commit = lookup_commit(oid.hash);
+		commit = lookup_commit(&oid);
 		if (!commit)
 			die("not a commit? can't happen: %s", oid_to_hex(&oid));
 
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 5f2c2ab23e..d4d573b985 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -636,8 +636,8 @@ static int update_local_ref(struct ref *ref,
 		return r;
 	}
 
-	current = lookup_commit_reference_gently(ref->old_oid.hash, 1);
-	updated = lookup_commit_reference_gently(ref->new_oid.hash, 1);
+	current = lookup_commit_reference_gently(&ref->old_oid, 1);
+	updated = lookup_commit_reference_gently(&ref->new_oid, 1);
 	if (!current || !updated) {
 		const char *msg;
 		const char *what;
@@ -770,7 +770,8 @@ static int store_updated_refs(const char *raw_url, const char *remote_name,
 				continue;
 			}
 
-			commit = lookup_commit_reference_gently(rm->old_oid.hash, 1);
+			commit = lookup_commit_reference_gently(&rm->old_oid,
+								1);
 			if (!commit)
 				rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;
 
diff --git a/builtin/fmt-merge-msg.c b/builtin/fmt-merge-msg.c
index 6faa3c0d24..70137b0b7e 100644
--- a/builtin/fmt-merge-msg.c
+++ b/builtin/fmt-merge-msg.c
@@ -341,7 +341,7 @@ static void shortlog(const char *name,
 	const struct object_id *oid = &origin_data->oid;
 	int limit = opts->shortlog_len;
 
-	branch = deref_tag(parse_object(oid->hash), oid_to_hex(oid), GIT_SHA1_HEXSZ);
+	branch = deref_tag(parse_object(oid), oid_to_hex(oid), GIT_SHA1_HEXSZ);
 	if (!branch || branch->type != OBJ_COMMIT)
 		return;
 
@@ -559,14 +559,14 @@ static void find_merge_parents(struct merge_parents *result,
 		 * "name" here and we do not want to contaminate its
 		 * util field yet.
 		 */
-		obj = parse_object(oid.hash);
+		obj = parse_object(&oid);
 		parent = (struct commit *)peel_to_type(NULL, 0, obj, OBJ_COMMIT);
 		if (!parent)
 			continue;
 		commit_list_insert(parent, &parents);
 		add_merge_parent(result, &obj->oid, &parent->object.oid);
 	}
-	head_commit = lookup_commit(head->hash);
+	head_commit = lookup_commit(head);
 	if (head_commit)
 		commit_list_insert(head_commit, &parents);
 	parents = reduce_heads(parents);
@@ -633,7 +633,7 @@ int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 		struct commit *head;
 		struct rev_info rev;
 
-		head = lookup_commit_or_die(head_oid.hash, "HEAD");
+		head = lookup_commit_or_die(&head_oid, "HEAD");
 		init_revisions(&rev, NULL);
 		rev.commit_format = CMIT_FMT_ONELINE;
 		rev.ignore_merges = 1;
diff --git a/builtin/fsck.c b/builtin/fsck.c
index 32a32e55c8..cb2ba6cd1b 100644
--- a/builtin/fsck.c
+++ b/builtin/fsck.c
@@ -377,7 +377,7 @@ static int fsck_obj(struct object *obj)
 	return 0;
 }
 
-static int fsck_obj_buffer(const unsigned char *sha1, enum object_type type,
+static int fsck_obj_buffer(const struct object_id *oid, enum object_type type,
 			   unsigned long size, void *buffer, int *eaten)
 {
 	/*
@@ -385,10 +385,10 @@ static int fsck_obj_buffer(const unsigned char *sha1, enum object_type type,
 	 * verify_packfile(), data_valid variable for details.
 	 */
 	struct object *obj;
-	obj = parse_object_buffer(sha1, type, size, buffer, eaten);
+	obj = parse_object_buffer(oid, type, size, buffer, eaten);
 	if (!obj) {
 		errors_found |= ERROR_OBJECT;
-		return error("%s: object corrupt or missing", sha1_to_hex(sha1));
+		return error("%s: object corrupt or missing", oid_to_hex(oid));
 	}
 	obj->flags = HAS_OBJ;
 	return fsck_obj(obj);
@@ -444,7 +444,7 @@ static int fsck_handle_ref(const char *refname, const struct object_id *oid,
 {
 	struct object *obj;
 
-	obj = parse_object(oid->hash);
+	obj = parse_object(oid);
 	if (!obj) {
 		error("%s: invalid sha1 pointer %s", refname, oid_to_hex(oid));
 		errors_found |= ERROR_REACHABLE;
@@ -506,7 +506,7 @@ static struct object *parse_loose_object(const struct object_id *oid,
 	if (!contents && type != OBJ_BLOB)
 		die("BUG: read_loose_object streamed a non-blob");
 
-	obj = parse_object_buffer(oid->hash, type, size, contents, &eaten);
+	obj = parse_object_buffer(oid, type, size, contents, &eaten);
 
 	if (!eaten)
 		free(contents);
@@ -599,10 +599,10 @@ static int fsck_cache_tree(struct cache_tree *it)
 		fprintf(stderr, "Checking cache tree\n");
 
 	if (0 <= it->entry_count) {
-		struct object *obj = parse_object(it->sha1);
+		struct object *obj = parse_object(&it->oid);
 		if (!obj) {
 			error("%s: invalid sha1 pointer in cache-tree",
-			      sha1_to_hex(it->sha1));
+			      oid_to_hex(&it->oid));
 			errors_found |= ERROR_REFS;
 			return 1;
 		}
@@ -781,7 +781,7 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 			mode = active_cache[i]->ce_mode;
 			if (S_ISGITLINK(mode))
 				continue;
-			blob = lookup_blob(active_cache[i]->oid.hash);
+			blob = lookup_blob(&active_cache[i]->oid);
 			if (!blob)
 				continue;
 			obj = &blob->object;
diff --git a/builtin/grep.c b/builtin/grep.c
index f2829629b6..c6c26e9b9e 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -1196,7 +1196,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			break;
 		}
 
-		object = parse_object_or_die(oid.hash, arg);
+		object = parse_object_or_die(&oid, arg);
 		if (!seen_dashdash)
 			verify_non_filename(prefix, arg);
 		add_object_array_with_path(object, arg, &list, oc.mode, oc.path);
diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 4ff567db47..04b9dcaf0f 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -747,13 +747,13 @@ static int compare_objects(const unsigned char *buf, unsigned long size,
 		ssize_t len = read_istream(data->st, data->buf, size);
 		if (len == 0)
 			die(_("SHA1 COLLISION FOUND WITH %s !"),
-			    sha1_to_hex(data->entry->idx.sha1));
+			    oid_to_hex(&data->entry->idx.oid));
 		if (len < 0)
 			die(_("unable to read %s"),
-			    sha1_to_hex(data->entry->idx.sha1));
+			    oid_to_hex(&data->entry->idx.oid));
 		if (memcmp(buf, data->buf, len))
 			die(_("SHA1 COLLISION FOUND WITH %s !"),
-			    sha1_to_hex(data->entry->idx.sha1));
+			    oid_to_hex(&data->entry->idx.oid));
 		size -= len;
 		buf += len;
 	}
@@ -771,12 +771,12 @@ static int check_collison(struct object_entry *entry)
 
 	memset(&data, 0, sizeof(data));
 	data.entry = entry;
-	data.st = open_istream(entry->idx.sha1, &type, &size, NULL);
+	data.st = open_istream(entry->idx.oid.hash, &type, &size, NULL);
 	if (!data.st)
 		return -1;
 	if (size != entry->size || type != entry->type)
 		die(_("SHA1 COLLISION FOUND WITH %s !"),
-		    sha1_to_hex(entry->idx.sha1));
+		    oid_to_hex(&entry->idx.oid));
 	unpack_data(entry, compare_objects, &data);
 	close_istream(data.st);
 	free(data.buf);
@@ -785,7 +785,7 @@ static int check_collison(struct object_entry *entry)
 
 static void sha1_object(const void *data, struct object_entry *obj_entry,
 			unsigned long size, enum object_type type,
-			const unsigned char *sha1)
+			const struct object_id *oid)
 {
 	void *new_data = NULL;
 	int collision_test_needed = 0;
@@ -794,7 +794,7 @@ static void sha1_object(const void *data, struct object_entry *obj_entry,
 
 	if (startup_info->have_repository) {
 		read_lock();
-		collision_test_needed = has_sha1_file_with_flags(sha1, HAS_SHA1_QUICK);
+		collision_test_needed = has_sha1_file_with_flags(oid->hash, HAS_SHA1_QUICK);
 		read_unlock();
 	}
 
@@ -809,31 +809,31 @@ static void sha1_object(const void *data, struct object_entry *obj_entry,
 		enum object_type has_type;
 		unsigned long has_size;
 		read_lock();
-		has_type = sha1_object_info(sha1, &has_size);
+		has_type = sha1_object_info(oid->hash, &has_size);
 		if (has_type < 0)
-			die(_("cannot read existing object info %s"), sha1_to_hex(sha1));
+			die(_("cannot read existing object info %s"), oid_to_hex(oid));
 		if (has_type != type || has_size != size)
-			die(_("SHA1 COLLISION FOUND WITH %s !"), sha1_to_hex(sha1));
-		has_data = read_sha1_file(sha1, &has_type, &has_size);
+			die(_("SHA1 COLLISION FOUND WITH %s !"), oid_to_hex(oid));
+		has_data = read_sha1_file(oid->hash, &has_type, &has_size);
 		read_unlock();
 		if (!data)
 			data = new_data = get_data_from_pack(obj_entry);
 		if (!has_data)
-			die(_("cannot read existing object %s"), sha1_to_hex(sha1));
+			die(_("cannot read existing object %s"), oid_to_hex(oid));
 		if (size != has_size || type != has_type ||
 		    memcmp(data, has_data, size) != 0)
-			die(_("SHA1 COLLISION FOUND WITH %s !"), sha1_to_hex(sha1));
+			die(_("SHA1 COLLISION FOUND WITH %s !"), oid_to_hex(oid));
 		free(has_data);
 	}
 
 	if (strict) {
 		read_lock();
 		if (type == OBJ_BLOB) {
-			struct blob *blob = lookup_blob(sha1);
+			struct blob *blob = lookup_blob(oid);
 			if (blob)
 				blob->object.flags |= FLAG_CHECKED;
 			else
-				die(_("invalid blob object %s"), sha1_to_hex(sha1));
+				die(_("invalid blob object %s"), oid_to_hex(oid));
 		} else {
 			struct object *obj;
 			int eaten;
@@ -845,7 +845,8 @@ static void sha1_object(const void *data, struct object_entry *obj_entry,
 			 * we do not need to free the memory here, as the
 			 * buf is deleted by the caller.
 			 */
-			obj = parse_object_buffer(sha1, type, size, buf, &eaten);
+			obj = parse_object_buffer(oid, type, size, buf,
+						  &eaten);
 			if (!obj)
 				die(_("invalid %s"), typename(type));
 			if (do_fsck_object &&
@@ -957,9 +958,10 @@ static void resolve_delta(struct object_entry *delta_obj,
 	if (!result->data)
 		bad_object(delta_obj->idx.offset, _("failed to apply delta"));
 	hash_sha1_file(result->data, result->size,
-		       typename(delta_obj->real_type), delta_obj->idx.sha1);
+		       typename(delta_obj->real_type),
+		       delta_obj->idx.oid.hash);
 	sha1_object(result->data, NULL, result->size, delta_obj->real_type,
-		    delta_obj->idx.sha1);
+		    &delta_obj->idx.oid);
 	counter_lock();
 	nr_resolved_deltas++;
 	counter_unlock();
@@ -989,7 +991,7 @@ static struct base_data *find_unresolved_deltas_1(struct base_data *base,
 						  struct base_data *prev_base)
 {
 	if (base->ref_last == -1 && base->ofs_last == -1) {
-		find_ref_delta_children(base->obj->idx.sha1,
+		find_ref_delta_children(base->obj->idx.oid.hash,
 					&base->ref_first, &base->ref_last,
 					OBJ_REF_DELTA);
 
@@ -1130,7 +1132,8 @@ static void parse_pack_objects(unsigned char *sha1)
 	for (i = 0; i < nr_objects; i++) {
 		struct object_entry *obj = &objects[i];
 		void *data = unpack_raw_entry(obj, &ofs_delta->offset,
-					      ref_delta_sha1, obj->idx.sha1);
+					      ref_delta_sha1,
+					      obj->idx.oid.hash);
 		obj->real_type = obj->type;
 		if (obj->type == OBJ_OFS_DELTA) {
 			nr_ofs_deltas++;
@@ -1146,7 +1149,8 @@ static void parse_pack_objects(unsigned char *sha1)
 			obj->real_type = OBJ_BAD;
 			nr_delays++;
 		} else
-			sha1_object(data, NULL, obj->size, obj->type, obj->idx.sha1);
+			sha1_object(data, NULL, obj->size, obj->type,
+				    &obj->idx.oid);
 		free(data);
 		display_progress(progress, i+1);
 	}
@@ -1172,7 +1176,8 @@ static void parse_pack_objects(unsigned char *sha1)
 		if (obj->real_type != OBJ_BAD)
 			continue;
 		obj->real_type = obj->type;
-		sha1_object(NULL, obj, obj->size, obj->type, obj->idx.sha1);
+		sha1_object(NULL, obj, obj->size, obj->type,
+			    &obj->idx.oid);
 		nr_delays--;
 	}
 	if (nr_delays)
@@ -1330,7 +1335,7 @@ static struct object_entry *append_obj_to_pack(struct sha1file *f,
 	obj[1].idx.offset += write_compressed(f, buf, size);
 	obj[0].idx.crc32 = crc32_end(f);
 	sha1flush(f);
-	hashcpy(obj->idx.sha1, sha1);
+	hashcpy(obj->idx.oid.hash, sha1);
 	return obj;
 }
 
@@ -1581,13 +1586,14 @@ static void show_pack_info(int stat_only)
 		if (stat_only)
 			continue;
 		printf("%s %-6s %lu %lu %"PRIuMAX,
-		       sha1_to_hex(obj->idx.sha1),
+		       oid_to_hex(&obj->idx.oid),
 		       typename(obj->real_type), obj->size,
 		       (unsigned long)(obj[1].idx.offset - obj->idx.offset),
 		       (uintmax_t)obj->idx.offset);
 		if (is_delta_type(obj->type)) {
 			struct object_entry *bobj = &objects[obj_stat[i].base_object_no];
-			printf(" %u %s", obj_stat[i].delta_depth, sha1_to_hex(bobj->idx.sha1));
+			printf(" %u %s", obj_stat[i].delta_depth,
+			       oid_to_hex(&bobj->idx.oid));
 		}
 		putchar('\n');
 	}
diff --git a/builtin/log.c b/builtin/log.c
index 631fbc984f..a440601efe 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -596,7 +596,7 @@ int cmd_show(int argc, const char **argv, const char *prefix)
 			rev.shown_one = 1;
 			if (ret)
 				break;
-			o = parse_object(t->tagged->oid.hash);
+			o = parse_object(&t->tagged->oid);
 			if (!o)
 				ret = error(_("Could not read object %s"),
 					    oid_to_hex(&t->tagged->oid));
@@ -878,8 +878,8 @@ static void get_patch_ids(struct rev_info *rev, struct patch_ids *ids)
 	o2 = rev->pending.objects[1].item;
 	flags1 = o1->flags;
 	flags2 = o2->flags;
-	c1 = lookup_commit_reference(o1->oid.hash);
-	c2 = lookup_commit_reference(o2->oid.hash);
+	c1 = lookup_commit_reference(&o1->oid);
+	c2 = lookup_commit_reference(&o2->oid);
 
 	if ((flags1 & UNINTERESTING) == (flags2 & UNINTERESTING))
 		die(_("Not a range."));
@@ -1263,7 +1263,7 @@ static struct commit *get_base_commit(const char *base_commit,
 
 			if (get_oid(upstream, &oid))
 				die(_("Failed to resolve '%s' as a valid ref."), upstream);
-			commit = lookup_commit_or_die(oid.hash, "upstream base");
+			commit = lookup_commit_or_die(&oid, "upstream base");
 			base_list = get_merge_bases_many(commit, total, list);
 			/* There should be one and only one merge base. */
 			if (!base_list || base_list->next)
@@ -1819,7 +1819,7 @@ static int add_pending_commit(const char *arg, struct rev_info *revs, int flags)
 {
 	struct object_id oid;
 	if (get_oid(arg, &oid) == 0) {
-		struct commit *commit = lookup_commit_reference(oid.hash);
+		struct commit *commit = lookup_commit_reference(&oid);
 		if (commit) {
 			commit->object.flags |= flags;
 			add_pending_object(revs, &commit->object, arg);
diff --git a/builtin/ls-files.c b/builtin/ls-files.c
index 61271b52cf..530e6ae7f7 100644
--- a/builtin/ls-files.c
+++ b/builtin/ls-files.c
@@ -414,14 +414,14 @@ static void prune_cache(const char *prefix, size_t prefixlen)
 void overlay_tree_on_cache(const char *tree_name, const char *prefix)
 {
 	struct tree *tree;
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct pathspec pathspec;
 	struct cache_entry *last_stage0 = NULL;
 	int i;
 
-	if (get_sha1(tree_name, sha1))
+	if (get_oid(tree_name, &oid))
 		die("tree-ish %s not found.", tree_name);
-	tree = parse_tree_indirect(sha1);
+	tree = parse_tree_indirect(&oid);
 	if (!tree)
 		die("bad tree-ish %s", tree_name);
 
diff --git a/builtin/ls-tree.c b/builtin/ls-tree.c
index d7ebeb4ce6..ee7b293b11 100644
--- a/builtin/ls-tree.c
+++ b/builtin/ls-tree.c
@@ -119,7 +119,7 @@ static int show_tree(const unsigned char *sha1, struct strbuf *base,
 
 int cmd_ls_tree(int argc, const char **argv, const char *prefix)
 {
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct tree *tree;
 	int i, full_tree = 0;
 	const struct option ls_tree_options[] = {
@@ -164,7 +164,7 @@ int cmd_ls_tree(int argc, const char **argv, const char *prefix)
 
 	if (argc < 1)
 		usage_with_options(ls_tree_usage, ls_tree_options);
-	if (get_sha1(argv[0], sha1))
+	if (get_oid(argv[0], &oid))
 		die("Not a valid object name %s", argv[0]);
 
 	/*
@@ -180,7 +180,7 @@ int cmd_ls_tree(int argc, const char **argv, const char *prefix)
 	for (i = 0; i < pathspec.nr; i++)
 		pathspec.items[i].nowildcard_len = pathspec.items[i].len;
 	pathspec.has_wildcard = 0;
-	tree = parse_tree_indirect(sha1);
+	tree = parse_tree_indirect(&oid);
 	if (!tree)
 		die("not a tree object");
 	return !!read_tree_recursive(tree, "", 0, 0, &pathspec, show_tree, NULL);
diff --git a/builtin/merge-base.c b/builtin/merge-base.c
index 8ed96391c1..0c36a70ad8 100644
--- a/builtin/merge-base.c
+++ b/builtin/merge-base.c
@@ -41,7 +41,7 @@ static struct commit *get_commit_reference(const char *arg)
 
 	if (get_oid(arg, &revkey))
 		die("Not a valid object name %s", arg);
-	r = lookup_commit_reference(revkey.hash);
+	r = lookup_commit_reference(&revkey);
 	if (!r)
 		die("Not a valid commit name %s", arg);
 
@@ -120,7 +120,7 @@ static void add_one_commit(struct object_id *oid, struct rev_collect *revs)
 	if (is_null_oid(oid))
 		return;
 
-	commit = lookup_commit(oid->hash);
+	commit = lookup_commit(oid);
 	if (!commit ||
 	    (commit->object.flags & TMP_MARK) ||
 	    parse_commit(commit))
@@ -168,7 +168,7 @@ static int handle_fork_point(int argc, const char **argv)
 	if (get_oid(commitname, &oid))
 		die("Not a valid object name: '%s'", commitname);
 
-	derived = lookup_commit_reference(oid.hash);
+	derived = lookup_commit_reference(&oid);
 	memset(&revs, 0, sizeof(revs));
 	revs.initial = 1;
 	for_each_reflog_ent(refname, collect_one_reflog_ent, &revs);
diff --git a/builtin/merge-tree.c b/builtin/merge-tree.c
index 5b7ab9b967..bad6735c76 100644
--- a/builtin/merge-tree.c
+++ b/builtin/merge-tree.c
@@ -161,14 +161,14 @@ static int both_empty(struct name_entry *a, struct name_entry *b)
 	return !(a->oid || b->oid);
 }
 
-static struct merge_list *create_entry(unsigned stage, unsigned mode, const unsigned char *sha1, const char *path)
+static struct merge_list *create_entry(unsigned stage, unsigned mode, const struct object_id *oid, const char *path)
 {
 	struct merge_list *res = xcalloc(1, sizeof(*res));
 
 	res->stage = stage;
 	res->path = path;
 	res->mode = mode;
-	res->blob = lookup_blob(sha1);
+	res->blob = lookup_blob(oid);
 	return res;
 }
 
@@ -188,8 +188,8 @@ static void resolve(const struct traverse_info *info, struct name_entry *ours, s
 		return;
 
 	path = traverse_path(info, result);
-	orig = create_entry(2, ours->mode, ours->oid->hash, path);
-	final = create_entry(0, result->mode, result->oid->hash, path);
+	orig = create_entry(2, ours->mode, ours->oid, path);
+	final = create_entry(0, result->mode, result->oid, path);
 
 	final->link = orig;
 
@@ -239,7 +239,7 @@ static struct merge_list *link_entry(unsigned stage, const struct traverse_info
 		path = entry->path;
 	else
 		path = traverse_path(info, n);
-	link = create_entry(stage, n->mode, n->oid->hash, path);
+	link = create_entry(stage, n->mode, n->oid, path);
 	link->link = entry;
 	return link;
 }
diff --git a/builtin/merge.c b/builtin/merge.c
index 703827f006..a4a098f40f 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -605,13 +605,13 @@ static int read_tree_trivial(struct object_id *common, struct object_id *head,
 	opts.verbose_update = 1;
 	opts.trivial_merges_only = 1;
 	opts.merge = 1;
-	trees[nr_trees] = parse_tree_indirect(common->hash);
+	trees[nr_trees] = parse_tree_indirect(common);
 	if (!trees[nr_trees++])
 		return -1;
-	trees[nr_trees] = parse_tree_indirect(head->hash);
+	trees[nr_trees] = parse_tree_indirect(head);
 	if (!trees[nr_trees++])
 		return -1;
-	trees[nr_trees] = parse_tree_indirect(one->hash);
+	trees[nr_trees] = parse_tree_indirect(one);
 	if (!trees[nr_trees++])
 		return -1;
 	opts.fn = threeway_merge;
@@ -1123,7 +1123,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	if (!branch || is_null_oid(&head_oid))
 		head_commit = NULL;
 	else
-		head_commit = lookup_commit_or_die(head_oid.hash, "HEAD");
+		head_commit = lookup_commit_or_die(&head_oid, "HEAD");
 
 	init_diff_ui_defaults();
 	git_config(git_merge_config, NULL);
@@ -1372,8 +1372,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			goto done;
 		}
 
-		if (checkout_fast_forward(head_commit->object.oid.hash,
-					  commit->object.oid.hash,
+		if (checkout_fast_forward(&head_commit->object.oid,
+					  &commit->object.oid,
 					  overwrite_ignore)) {
 			ret = 1;
 			goto done;
diff --git a/builtin/name-rev.c b/builtin/name-rev.c
index 4437475017..1bf4cc4ebb 100644
--- a/builtin/name-rev.c
+++ b/builtin/name-rev.c
@@ -114,7 +114,7 @@ struct name_ref_data {
 
 static struct tip_table {
 	struct tip_table_entry {
-		unsigned char sha1[20];
+		struct object_id oid;
 		const char *refname;
 	} *table;
 	int nr;
@@ -122,13 +122,13 @@ static struct tip_table {
 	int sorted;
 } tip_table;
 
-static void add_to_tip_table(const unsigned char *sha1, const char *refname,
+static void add_to_tip_table(const struct object_id *oid, const char *refname,
 			     int shorten_unambiguous)
 {
 	refname = name_ref_abbrev(refname, shorten_unambiguous);
 
 	ALLOC_GROW(tip_table.table, tip_table.nr + 1, tip_table.alloc);
-	hashcpy(tip_table.table[tip_table.nr].sha1, sha1);
+	oidcpy(&tip_table.table[tip_table.nr].oid, oid);
 	tip_table.table[tip_table.nr].refname = xstrdup(refname);
 	tip_table.nr++;
 	tip_table.sorted = 0;
@@ -137,12 +137,12 @@ static void add_to_tip_table(const unsigned char *sha1, const char *refname,
 static int tipcmp(const void *a_, const void *b_)
 {
 	const struct tip_table_entry *a = a_, *b = b_;
-	return hashcmp(a->sha1, b->sha1);
+	return oidcmp(&a->oid, &b->oid);
 }
 
 static int name_ref(const char *path, const struct object_id *oid, int flags, void *cb_data)
 {
-	struct object *o = parse_object(oid->hash);
+	struct object *o = parse_object(oid);
 	struct name_ref_data *data = cb_data;
 	int can_abbreviate_output = data->tags_only && data->name_only;
 	int deref = 0;
@@ -194,13 +194,13 @@ static int name_ref(const char *path, const struct object_id *oid, int flags, vo
 			return 0;
 	}
 
-	add_to_tip_table(oid->hash, path, can_abbreviate_output);
+	add_to_tip_table(oid, path, can_abbreviate_output);
 
 	while (o && o->type == OBJ_TAG) {
 		struct tag *t = (struct tag *) o;
 		if (!t->tagged)
 			break; /* broken repository */
-		o = parse_object(t->tagged->oid.hash);
+		o = parse_object(&t->tagged->oid);
 		deref = 1;
 		taggerdate = t->date;
 	}
@@ -216,7 +216,7 @@ static int name_ref(const char *path, const struct object_id *oid, int flags, vo
 static const unsigned char *nth_tip_table_ent(size_t ix, void *table_)
 {
 	struct tip_table_entry *table = table_;
-	return table[ix].sha1;
+	return table[ix].oid.hash;
 }
 
 static const char *get_exact_ref_match(const struct object *o)
@@ -301,9 +301,9 @@ static void name_rev_line(char *p, struct name_ref_data *data)
 #define ishex(x) (isdigit((x)) || ((x) >= 'a' && (x) <= 'f'))
 		if (!ishex(*p))
 			forty = 0;
-		else if (++forty == 40 &&
+		else if (++forty == GIT_SHA1_HEXSZ &&
 			 !ishex(*(p+1))) {
-			unsigned char sha1[40];
+			struct object_id oid;
 			const char *name = NULL;
 			char c = *(p+1);
 			int p_len = p - p_start + 1;
@@ -311,9 +311,9 @@ static void name_rev_line(char *p, struct name_ref_data *data)
 			forty = 0;
 
 			*(p+1) = 0;
-			if (!get_sha1(p - 39, sha1)) {
+			if (!get_oid(p - (GIT_SHA1_HEXSZ - 1), &oid)) {
 				struct object *o =
-					lookup_object(sha1);
+					lookup_object(oid.hash);
 				if (o)
 					name = get_rev_name(o, &buf);
 			}
@@ -323,7 +323,7 @@ static void name_rev_line(char *p, struct name_ref_data *data)
 				continue;
 
 			if (data->name_only)
-				printf("%.*s%s", p_len - 40, p_start, name);
+				printf("%.*s%s", p_len - GIT_SHA1_HEXSZ, p_start, name);
 			else
 				printf("%.*s (%s)", p_len, p_start, name);
 			p_start = p + 1;
@@ -374,18 +374,18 @@ int cmd_name_rev(int argc, const char **argv, const char *prefix)
 		cutoff = 0;
 
 	for (; argc; argc--, argv++) {
-		unsigned char sha1[20];
+		struct object_id oid;
 		struct object *object;
 		struct commit *commit;
 
-		if (get_sha1(*argv, sha1)) {
+		if (get_oid(*argv, &oid)) {
 			fprintf(stderr, "Could not get sha1 for %s. Skipping.\n",
 					*argv);
 			continue;
 		}
 
 		commit = NULL;
-		object = parse_object(sha1);
+		object = parse_object(&oid);
 		if (object) {
 			struct object *peeled = deref_tag(object, *argv, 0);
 			if (peeled && peeled->type == OBJ_COMMIT)
diff --git a/builtin/notes.c b/builtin/notes.c
index 7b891471c4..f2847c41e0 100644
--- a/builtin/notes.c
+++ b/builtin/notes.c
@@ -706,7 +706,7 @@ static int merge_commit(struct notes_merge_options *o)
 
 	if (get_oid("NOTES_MERGE_PARTIAL", &oid))
 		die(_("failed to read ref NOTES_MERGE_PARTIAL"));
-	else if (!(partial = lookup_commit_reference(oid.hash)))
+	else if (!(partial = lookup_commit_reference(&oid)))
 		die(_("could not find commit from NOTES_MERGE_PARTIAL."));
 	else if (parse_commit(partial))
 		die(_("could not parse commit from NOTES_MERGE_PARTIAL."));
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 9b4ba8a80d..82555e410a 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -106,12 +106,14 @@ static void *get_delta(struct object_entry *entry)
 	void *buf, *base_buf, *delta_buf;
 	enum object_type type;
 
-	buf = read_sha1_file(entry->idx.sha1, &type, &size);
+	buf = read_sha1_file(entry->idx.oid.hash, &type, &size);
 	if (!buf)
-		die("unable to read %s", sha1_to_hex(entry->idx.sha1));
-	base_buf = read_sha1_file(entry->delta->idx.sha1, &type, &base_size);
+		die("unable to read %s", oid_to_hex(&entry->idx.oid));
+	base_buf = read_sha1_file(entry->delta->idx.oid.hash, &type,
+				  &base_size);
 	if (!base_buf)
-		die("unable to read %s", sha1_to_hex(entry->delta->idx.sha1));
+		die("unable to read %s",
+		    oid_to_hex(&entry->delta->idx.oid));
 	delta_buf = diff_delta(base_buf, base_size,
 			       buf, size, &delta_size, 0);
 	if (!delta_buf || delta_size != entry->delta_size)
@@ -249,12 +251,14 @@ static unsigned long write_no_reuse_object(struct sha1file *f, struct object_ent
 	if (!usable_delta) {
 		if (entry->type == OBJ_BLOB &&
 		    entry->size > big_file_threshold &&
-		    (st = open_istream(entry->idx.sha1, &type, &size, NULL)) != NULL)
+		    (st = open_istream(entry->idx.oid.hash, &type, &size, NULL)) != NULL)
 			buf = NULL;
 		else {
-			buf = read_sha1_file(entry->idx.sha1, &type, &size);
+			buf = read_sha1_file(entry->idx.oid.hash, &type,
+					     &size);
 			if (!buf)
-				die(_("unable to read %s"), sha1_to_hex(entry->idx.sha1));
+				die(_("unable to read %s"),
+				    oid_to_hex(&entry->idx.oid));
 		}
 		/*
 		 * make sure no cached delta data remains from a
@@ -322,7 +326,7 @@ static unsigned long write_no_reuse_object(struct sha1file *f, struct object_ent
 			return 0;
 		}
 		sha1write(f, header, hdrlen);
-		sha1write(f, entry->delta->idx.sha1, 20);
+		sha1write(f, entry->delta->idx.oid.hash, 20);
 		hdrlen += 20;
 	} else {
 		if (limit && hdrlen + datalen + 20 >= limit) {
@@ -334,7 +338,7 @@ static unsigned long write_no_reuse_object(struct sha1file *f, struct object_ent
 		sha1write(f, header, hdrlen);
 	}
 	if (st) {
-		datalen = write_large_blob_data(st, f, entry->idx.sha1);
+		datalen = write_large_blob_data(st, f, entry->idx.oid.hash);
 		close_istream(st);
 	} else {
 		sha1write(f, buf, datalen);
@@ -369,7 +373,8 @@ static off_t write_reuse_object(struct sha1file *f, struct object_entry *entry,
 	datalen = revidx[1].offset - offset;
 	if (!pack_to_stdout && p->index_version > 1 &&
 	    check_pack_crc(p, &w_curs, offset, datalen, revidx->nr)) {
-		error("bad packed object CRC for %s", sha1_to_hex(entry->idx.sha1));
+		error("bad packed object CRC for %s",
+		      oid_to_hex(&entry->idx.oid));
 		unuse_pack(&w_curs);
 		return write_no_reuse_object(f, entry, limit, usable_delta);
 	}
@@ -379,7 +384,8 @@ static off_t write_reuse_object(struct sha1file *f, struct object_entry *entry,
 
 	if (!pack_to_stdout && p->index_version == 1 &&
 	    check_pack_inflate(p, &w_curs, offset, datalen, entry->size)) {
-		error("corrupt packed object for %s", sha1_to_hex(entry->idx.sha1));
+		error("corrupt packed object for %s",
+		      oid_to_hex(&entry->idx.oid));
 		unuse_pack(&w_curs);
 		return write_no_reuse_object(f, entry, limit, usable_delta);
 	}
@@ -404,7 +410,7 @@ static off_t write_reuse_object(struct sha1file *f, struct object_entry *entry,
 			return 0;
 		}
 		sha1write(f, header, hdrlen);
-		sha1write(f, entry->delta->idx.sha1, 20);
+		sha1write(f, entry->delta->idx.oid.hash, 20);
 		hdrlen += 20;
 		reused_delta++;
 	} else {
@@ -509,7 +515,7 @@ static enum write_one_status write_one(struct sha1file *f,
 	recursing = (e->idx.offset == 1);
 	if (recursing) {
 		warning("recursive delta detected for object %s",
-			sha1_to_hex(e->idx.sha1));
+			oid_to_hex(&e->idx.oid));
 		return WRITE_ONE_RECURSIVE;
 	} else if (e->idx.offset || e->preferred_base) {
 		/* offset is non zero if object is written already. */
@@ -1432,7 +1438,7 @@ static void check_object(struct object_entry *entry)
 				ofs += 1;
 				if (!ofs || MSB(ofs, 7)) {
 					error("delta base offset overflow in pack for %s",
-					      sha1_to_hex(entry->idx.sha1));
+					      oid_to_hex(&entry->idx.oid));
 					goto give_up;
 				}
 				c = buf[used_0++];
@@ -1441,7 +1447,7 @@ static void check_object(struct object_entry *entry)
 			ofs = entry->in_pack_offset - ofs;
 			if (ofs <= 0 || ofs >= entry->in_pack_offset) {
 				error("delta base offset out of bound for %s",
-				      sha1_to_hex(entry->idx.sha1));
+				      oid_to_hex(&entry->idx.oid));
 				goto give_up;
 			}
 			if (reuse_delta && !entry->preferred_base) {
@@ -1498,7 +1504,7 @@ static void check_object(struct object_entry *entry)
 		unuse_pack(&w_curs);
 	}
 
-	entry->type = sha1_object_info(entry->idx.sha1, &entry->size);
+	entry->type = sha1_object_info(entry->idx.oid.hash, &entry->size);
 	/*
 	 * The error condition is checked in prepare_pack().  This is
 	 * to permit a missing preferred base object to be ignored
@@ -1514,7 +1520,7 @@ static int pack_offset_sort(const void *_a, const void *_b)
 
 	/* avoid filesystem trashing with loose objects */
 	if (!a->in_pack && !b->in_pack)
-		return hashcmp(a->idx.sha1, b->idx.sha1);
+		return oidcmp(&a->idx.oid, &b->idx.oid);
 
 	if (a->in_pack < b->in_pack)
 		return -1;
@@ -1560,7 +1566,8 @@ static void drop_reused_delta(struct object_entry *entry)
 		 * And if that fails, the error will be recorded in entry->type
 		 * and dealt with in prepare_pack().
 		 */
-		entry->type = sha1_object_info(entry->idx.sha1, &entry->size);
+		entry->type = sha1_object_info(entry->idx.oid.hash,
+					       &entry->size);
 	}
 }
 
@@ -1852,26 +1859,29 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
 	/* Load data if not already done */
 	if (!trg->data) {
 		read_lock();
-		trg->data = read_sha1_file(trg_entry->idx.sha1, &type, &sz);
+		trg->data = read_sha1_file(trg_entry->idx.oid.hash, &type,
+					   &sz);
 		read_unlock();
 		if (!trg->data)
 			die("object %s cannot be read",
-			    sha1_to_hex(trg_entry->idx.sha1));
+			    oid_to_hex(&trg_entry->idx.oid));
 		if (sz != trg_size)
 			die("object %s inconsistent object length (%lu vs %lu)",
-			    sha1_to_hex(trg_entry->idx.sha1), sz, trg_size);
+			    oid_to_hex(&trg_entry->idx.oid), sz,
+			    trg_size);
 		*mem_usage += sz;
 	}
 	if (!src->data) {
 		read_lock();
-		src->data = read_sha1_file(src_entry->idx.sha1, &type, &sz);
+		src->data = read_sha1_file(src_entry->idx.oid.hash, &type,
+					   &sz);
 		read_unlock();
 		if (!src->data) {
 			if (src_entry->preferred_base) {
 				static int warned = 0;
 				if (!warned++)
 					warning("object %s cannot be read",
-						sha1_to_hex(src_entry->idx.sha1));
+						oid_to_hex(&src_entry->idx.oid));
 				/*
 				 * Those objects are not included in the
 				 * resulting pack.  Be resilient and ignore
@@ -1881,11 +1891,12 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
 				return 0;
 			}
 			die("object %s cannot be read",
-			    sha1_to_hex(src_entry->idx.sha1));
+			    oid_to_hex(&src_entry->idx.oid));
 		}
 		if (sz != src_size)
 			die("object %s inconsistent object length (%lu vs %lu)",
-			    sha1_to_hex(src_entry->idx.sha1), sz, src_size);
+			    oid_to_hex(&src_entry->idx.oid), sz,
+			    src_size);
 		*mem_usage += sz;
 	}
 	if (!src->index) {
@@ -2337,7 +2348,7 @@ static void add_tag_chain(const struct object_id *oid)
 	if (packlist_find(&to_pack, oid->hash, NULL))
 		return;
 
-	tag = lookup_tag(oid->hash);
+	tag = lookup_tag(oid);
 	while (1) {
 		if (!tag || parse_tag(tag) || !tag->tagged)
 			die("unable to pack objects reachable from tag %s",
@@ -2406,7 +2417,7 @@ static void prepare_pack(int window, int depth)
 			nr_deltas++;
 			if (entry->type < 0)
 				die("unable to get type of object %s",
-				    sha1_to_hex(entry->idx.sha1));
+				    oid_to_hex(&entry->idx.oid));
 		} else {
 			if (entry->type < 0) {
 				/*
@@ -2777,10 +2788,10 @@ static void get_object_list(int ac, const char **av)
 				continue;
 			}
 			if (starts_with(line, "--shallow ")) {
-				unsigned char sha1[20];
-				if (get_sha1_hex(line + 10, sha1))
+				struct object_id oid;
+				if (get_oid_hex(line + 10, &oid))
 					die("not an SHA-1 '%s'", line + 10);
-				register_shallow(sha1);
+				register_shallow(&oid);
 				use_bitmap_index = 0;
 				continue;
 			}
diff --git a/builtin/prune.c b/builtin/prune.c
index 8dcfecde0f..f0e2bff04c 100644
--- a/builtin/prune.c
+++ b/builtin/prune.c
@@ -123,11 +123,12 @@ int cmd_prune(int argc, const char **argv, const char *prefix)
 		die(_("cannot prune in a precious-objects repo"));
 
 	while (argc--) {
-		unsigned char sha1[20];
+		struct object_id oid;
 		const char *name = *argv++;
 
-		if (!get_sha1(name, sha1)) {
-			struct object *object = parse_object_or_die(sha1, name);
+		if (!get_oid(name, &oid)) {
+			struct object *object = parse_object_or_die(&oid,
+								    name);
 			add_pending_object(&revs, object, "");
 		}
 		else
diff --git a/builtin/pull.c b/builtin/pull.c
index dd1a4a94e4..318c273eb3 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -523,7 +523,7 @@ static int pull_into_void(const struct object_id *merge_head,
 	 * index/worktree changes that the user already made on the unborn
 	 * branch.
 	 */
-	if (checkout_fast_forward(EMPTY_TREE_SHA1_BIN, merge_head->hash, 0))
+	if (checkout_fast_forward(&empty_tree_oid, merge_head, 0))
 		return 1;
 
 	if (update_ref("initial pull", "HEAD", merge_head->hash, curr_head->hash, 0, UPDATE_REFS_DIE_ON_ERR))
@@ -698,10 +698,10 @@ static int get_octopus_merge_base(struct object_id *merge_base,
 {
 	struct commit_list *revs = NULL, *result;
 
-	commit_list_insert(lookup_commit_reference(curr_head->hash), &revs);
-	commit_list_insert(lookup_commit_reference(merge_head->hash), &revs);
+	commit_list_insert(lookup_commit_reference(curr_head), &revs);
+	commit_list_insert(lookup_commit_reference(merge_head), &revs);
 	if (!is_null_oid(fork_point))
-		commit_list_insert(lookup_commit_reference(fork_point->hash), &revs);
+		commit_list_insert(lookup_commit_reference(fork_point), &revs);
 
 	result = reduce_heads(get_octopus_merge_bases(revs));
 	free_commit_list(revs);
@@ -839,7 +839,7 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 			"fast-forwarding your working tree from\n"
 			"commit %s."), oid_to_hex(&orig_head));
 
-		if (checkout_fast_forward(orig_head.hash, curr_head.hash, 0))
+		if (checkout_fast_forward(&orig_head, &curr_head, 0))
 			die(_("Cannot fast-forward your working tree.\n"
 				"After making sure that you saved anything precious from\n"
 				"$ git diff %s\n"
@@ -865,9 +865,9 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 		struct commit_list *list = NULL;
 		struct commit *merge_head, *head;
 
-		head = lookup_commit_reference(orig_head.hash);
+		head = lookup_commit_reference(&orig_head);
 		commit_list_insert(head, &list);
-		merge_head = lookup_commit_reference(merge_heads.oid[0].hash);
+		merge_head = lookup_commit_reference(&merge_heads.oid[0]);
 		if (is_descendant_of(merge_head, list)) {
 			/* we can fast-forward this without invoking rebase */
 			opt_ff = "--ff-only";
diff --git a/builtin/read-tree.c b/builtin/read-tree.c
index 23e212ee8c..6d45175f6a 100644
--- a/builtin/read-tree.c
+++ b/builtin/read-tree.c
@@ -23,13 +23,13 @@ static int read_empty;
 static struct tree *trees[MAX_UNPACK_TREES];
 static int recurse_submodules = RECURSE_SUBMODULES_DEFAULT;
 
-static int list_tree(unsigned char *sha1)
+static int list_tree(struct object_id *oid)
 {
 	struct tree *tree;
 
 	if (nr_trees >= MAX_UNPACK_TREES)
 		die("I cannot read more than %d trees", MAX_UNPACK_TREES);
-	tree = parse_tree_indirect(sha1);
+	tree = parse_tree_indirect(oid);
 	if (!tree)
 		return -1;
 	trees[nr_trees++] = tree;
@@ -121,7 +121,7 @@ static struct lock_file lock_file;
 int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 {
 	int i, stage = 0;
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct tree_desc t[MAX_UNPACK_TREES];
 	struct unpack_trees_options opts;
 	int prefix_set = 0;
@@ -204,9 +204,9 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 	for (i = 0; i < argc; i++) {
 		const char *arg = argv[i];
 
-		if (get_sha1(arg, sha1))
+		if (get_oid(arg, &oid))
 			die("Not a valid object name %s", arg);
-		if (list_tree(sha1) < 0)
+		if (list_tree(&oid) < 0)
 			die("failed to unpack tree object %s", arg);
 		stage++;
 	}
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 1b29975c52..eb8b64bfce 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -900,7 +900,7 @@ static int update_shallow_ref(struct command *cmd, struct shallow_info *si)
 	 * not lose these new roots..
 	 */
 	for (i = 0; i < extra.nr; i++)
-		register_shallow(extra.oid[i].hash);
+		register_shallow(&extra.oid[i]);
 
 	si->shallow_ref[cmd->index] = 0;
 	oid_array_clear(&extra);
@@ -1100,8 +1100,8 @@ static const char *update(struct command *cmd, struct shallow_info *si)
 		struct object *old_object, *new_object;
 		struct commit *old_commit, *new_commit;
 
-		old_object = parse_object(old_oid->hash);
-		new_object = parse_object(new_oid->hash);
+		old_object = parse_object(old_oid);
+		new_object = parse_object(new_oid);
 
 		if (!old_object || !new_object ||
 		    old_object->type != OBJ_COMMIT ||
@@ -1124,7 +1124,7 @@ static const char *update(struct command *cmd, struct shallow_info *si)
 
 	if (is_null_oid(new_oid)) {
 		struct strbuf err = STRBUF_INIT;
-		if (!parse_object(old_oid->hash)) {
+		if (!parse_object(old_oid)) {
 			old_oid = NULL;
 			if (ref_exists(name)) {
 				rp_warning("Allowing deletion of corrupt ref.");
diff --git a/builtin/reflog.c b/builtin/reflog.c
index 4228d9ff4d..920c16dac0 100644
--- a/builtin/reflog.c
+++ b/builtin/reflog.c
@@ -55,14 +55,14 @@ struct collect_reflog_cb {
 #define STUDYING	(1u<<11)
 #define REACHABLE	(1u<<12)
 
-static int tree_is_complete(const unsigned char *sha1)
+static int tree_is_complete(const struct object_id *oid)
 {
 	struct tree_desc desc;
 	struct name_entry entry;
 	int complete;
 	struct tree *tree;
 
-	tree = lookup_tree(sha1);
+	tree = lookup_tree(oid);
 	if (!tree)
 		return 0;
 	if (tree->object.flags & SEEN)
@@ -73,7 +73,7 @@ static int tree_is_complete(const unsigned char *sha1)
 	if (!tree->buffer) {
 		enum object_type type;
 		unsigned long size;
-		void *data = read_sha1_file(sha1, &type, &size);
+		void *data = read_sha1_file(oid->hash, &type, &size);
 		if (!data) {
 			tree->object.flags |= INCOMPLETE;
 			return 0;
@@ -85,7 +85,7 @@ static int tree_is_complete(const unsigned char *sha1)
 	complete = 1;
 	while (tree_entry(&desc, &entry)) {
 		if (!has_sha1_file(entry.oid->hash) ||
-		    (S_ISDIR(entry.mode) && !tree_is_complete(entry.oid->hash))) {
+		    (S_ISDIR(entry.mode) && !tree_is_complete(entry.oid))) {
 			tree->object.flags |= INCOMPLETE;
 			complete = 0;
 		}
@@ -126,7 +126,7 @@ static int commit_is_complete(struct commit *commit)
 		struct commit_list *parent;
 
 		c = (struct commit *)study.objects[--study.nr].item;
-		if (!c->object.parsed && !parse_object(c->object.oid.hash))
+		if (!c->object.parsed && !parse_object(&c->object.oid))
 			c->object.flags |= INCOMPLETE;
 
 		if (c->object.flags & INCOMPLETE) {
@@ -152,7 +152,7 @@ static int commit_is_complete(struct commit *commit)
 		for (i = 0; i < found.nr; i++) {
 			struct commit *c =
 				(struct commit *)found.objects[i].item;
-			if (!tree_is_complete(c->tree->object.oid.hash)) {
+			if (!tree_is_complete(&c->tree->object.oid)) {
 				is_incomplete = 1;
 				c->object.flags |= INCOMPLETE;
 			}
@@ -186,13 +186,13 @@ static int commit_is_complete(struct commit *commit)
 	return !is_incomplete;
 }
 
-static int keep_entry(struct commit **it, unsigned char *sha1)
+static int keep_entry(struct commit **it, struct object_id *oid)
 {
 	struct commit *commit;
 
-	if (is_null_sha1(sha1))
+	if (is_null_oid(oid))
 		return 1;
-	commit = lookup_commit_reference_gently(sha1, 1);
+	commit = lookup_commit_reference_gently(oid, 1);
 	if (!commit)
 		return 0;
 
@@ -251,17 +251,17 @@ static void mark_reachable(struct expire_reflog_policy_cb *cb)
 	cb->mark_list = leftover;
 }
 
-static int unreachable(struct expire_reflog_policy_cb *cb, struct commit *commit, unsigned char *sha1)
+static int unreachable(struct expire_reflog_policy_cb *cb, struct commit *commit, struct object_id *oid)
 {
 	/*
 	 * We may or may not have the commit yet - if not, look it
 	 * up using the supplied sha1.
 	 */
 	if (!commit) {
-		if (is_null_sha1(sha1))
+		if (is_null_oid(oid))
 			return 0;
 
-		commit = lookup_commit_reference_gently(sha1, 1);
+		commit = lookup_commit_reference_gently(oid, 1);
 
 		/* Not a commit -- keep it */
 		if (!commit)
@@ -283,7 +283,7 @@ static int unreachable(struct expire_reflog_policy_cb *cb, struct commit *commit
 /*
  * Return true iff the specified reflog entry should be expired.
  */
-static int should_expire_reflog_ent(unsigned char *osha1, unsigned char *nsha1,
+static int should_expire_reflog_ent(struct object_id *ooid, struct object_id *noid,
 				    const char *email, timestamp_t timestamp, int tz,
 				    const char *message, void *cb_data)
 {
@@ -295,13 +295,13 @@ static int should_expire_reflog_ent(unsigned char *osha1, unsigned char *nsha1,
 
 	old = new = NULL;
 	if (cb->cmd.stalefix &&
-	    (!keep_entry(&old, osha1) || !keep_entry(&new, nsha1)))
+	    (!keep_entry(&old, ooid) || !keep_entry(&new, noid)))
 		return 1;
 
 	if (timestamp < cb->cmd.expire_unreachable) {
 		if (cb->unreachable_expire_kind == UE_ALWAYS)
 			return 1;
-		if (unreachable(cb, old, osha1) || unreachable(cb, new, nsha1))
+		if (unreachable(cb, old, ooid) || unreachable(cb, new, noid))
 			return 1;
 	}
 
@@ -318,7 +318,7 @@ static int push_tip_to_list(const char *refname, const struct object_id *oid,
 	struct commit *tip_commit;
 	if (flags & REF_ISSYMREF)
 		return 0;
-	tip_commit = lookup_commit_reference_gently(oid->hash, 1);
+	tip_commit = lookup_commit_reference_gently(oid, 1);
 	if (!tip_commit)
 		return 0;
 	commit_list_insert(tip_commit, list);
@@ -326,7 +326,7 @@ static int push_tip_to_list(const char *refname, const struct object_id *oid,
 }
 
 static void reflog_expiry_prepare(const char *refname,
-				  const unsigned char *sha1,
+				  const struct object_id *oid,
 				  void *cb_data)
 {
 	struct expire_reflog_policy_cb *cb = cb_data;
@@ -335,7 +335,7 @@ static void reflog_expiry_prepare(const char *refname,
 		cb->tip_commit = NULL;
 		cb->unreachable_expire_kind = UE_HEAD;
 	} else {
-		cb->tip_commit = lookup_commit_reference_gently(sha1, 1);
+		cb->tip_commit = lookup_commit_reference_gently(oid, 1);
 		if (!cb->tip_commit)
 			cb->unreachable_expire_kind = UE_ALWAYS;
 		else
diff --git a/builtin/replace.c b/builtin/replace.c
index ab17668f43..c921bc976f 100644
--- a/builtin/replace.c
+++ b/builtin/replace.c
@@ -328,7 +328,7 @@ static void replace_parents(struct strbuf *buf, int argc, const char **argv)
 		struct object_id oid;
 		if (get_oid(argv[i], &oid) < 0)
 			die(_("Not a valid object name: '%s'"), argv[i]);
-		lookup_commit_or_die(oid.hash, argv[i]);
+		lookup_commit_or_die(&oid, argv[i]);
 		strbuf_addf(&new_parents, "parent %s\n", oid_to_hex(&oid));
 	}
 
@@ -355,7 +355,7 @@ static void check_one_mergetag(struct commit *commit,
 	int i;
 
 	hash_sha1_file(extra->value, extra->len, typename(OBJ_TAG), tag_oid.hash);
-	tag = lookup_tag(tag_oid.hash);
+	tag = lookup_tag(&tag_oid);
 	if (!tag)
 		die(_("bad mergetag in commit '%s'"), ref);
 	if (parse_tag_buffer(tag, extra->value, extra->len))
@@ -394,7 +394,7 @@ static int create_graft(int argc, const char **argv, int force)
 
 	if (get_oid(old_ref, &old) < 0)
 		die(_("Not a valid object name: '%s'"), old_ref);
-	commit = lookup_commit_or_die(old.hash, old_ref);
+	commit = lookup_commit_or_die(&old, old_ref);
 
 	buffer = get_commit_buffer(commit, &size);
 	strbuf_add(&buf, buffer, size);
diff --git a/builtin/reset.c b/builtin/reset.c
index 5ce27fcaed..0afe1df250 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -105,7 +105,7 @@ static int reset_index(const struct object_id *oid, int reset_type, int quiet)
 		return -1;
 
 	if (reset_type == MIXED || reset_type == HARD) {
-		tree = parse_tree_indirect(oid->hash);
+		tree = parse_tree_indirect(oid);
 		prime_cache_tree(&the_index, tree);
 	}
 
@@ -175,7 +175,7 @@ static int read_from_tree(const struct pathspec *pathspec,
 	opt.format_callback = update_index_from_diff;
 	opt.format_callback_data = &intent_to_add;
 
-	if (do_diff_cache(tree_oid->hash, &opt))
+	if (do_diff_cache(tree_oid, &opt))
 		return 1;
 	diffcore_std(&opt);
 	diff_flush(&opt);
@@ -333,7 +333,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 		struct commit *commit;
 		if (get_sha1_committish(rev, oid.hash))
 			die(_("Failed to resolve '%s' as a valid revision."), rev);
-		commit = lookup_commit_reference(oid.hash);
+		commit = lookup_commit_reference(&oid);
 		if (!commit)
 			die(_("Could not parse object '%s'."), rev);
 		oidcpy(&oid, &commit->object.oid);
@@ -341,7 +341,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 		struct tree *tree;
 		if (get_sha1_treeish(rev, oid.hash))
 			die(_("Failed to resolve '%s' as a valid tree."), rev);
-		tree = parse_tree_indirect(oid.hash);
+		tree = parse_tree_indirect(&oid);
 		if (!tree)
 			die(_("Could not parse object '%s'."), rev);
 		oidcpy(&oid, &tree->object.oid);
@@ -410,7 +410,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 		update_ref_status = reset_refs(rev, &oid);
 
 		if (reset_type == HARD && !update_ref_status && !quiet)
-			print_new_head_line(lookup_commit_reference(oid.hash));
+			print_new_head_line(lookup_commit_reference(&oid));
 	}
 	if (!pathspec.nr)
 		remove_branch_state();
diff --git a/builtin/rev-list.c b/builtin/rev-list.c
index 3b292c99bd..718c6059c9 100644
--- a/builtin/rev-list.c
+++ b/builtin/rev-list.c
@@ -181,7 +181,7 @@ static void finish_object(struct object *obj, const char *name, void *cb_data)
 	if (obj->type == OBJ_BLOB && !has_object_file(&obj->oid))
 		die("missing blob object '%s'", oid_to_hex(&obj->oid));
 	if (info->revs->verify_objects && !obj->parsed && obj->type != OBJ_COMMIT)
-		parse_object(obj->oid.hash);
+		parse_object(&obj->oid);
 }
 
 static void show_object(struct object *obj, const char *name, void *cb_data)
diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index b450900243..efdc14473b 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -121,7 +121,7 @@ static void show_with_type(int type, const char *arg)
 }
 
 /* Output a revision, only if filter allows it */
-static void show_rev(int type, const unsigned char *sha1, const char *name)
+static void show_rev(int type, const struct object_id *oid, const char *name)
 {
 	if (!(filter & DO_REVS))
 		return;
@@ -129,10 +129,10 @@ static void show_rev(int type, const unsigned char *sha1, const char *name)
 
 	if ((symbolic || abbrev_ref) && name) {
 		if (symbolic == SHOW_SYMBOLIC_FULL || abbrev_ref) {
-			unsigned char discard[20];
+			struct object_id discard;
 			char *full;
 
-			switch (dwim_ref(name, strlen(name), discard, &full)) {
+			switch (dwim_ref(name, strlen(name), discard.hash, &full)) {
 			case 0:
 				/*
 				 * Not found -- not a ref.  We could
@@ -158,9 +158,9 @@ static void show_rev(int type, const unsigned char *sha1, const char *name)
 		}
 	}
 	else if (abbrev)
-		show_with_type(type, find_unique_abbrev(sha1, abbrev));
+		show_with_type(type, find_unique_abbrev(oid->hash, abbrev));
 	else
-		show_with_type(type, sha1_to_hex(sha1));
+		show_with_type(type, oid_to_hex(oid));
 }
 
 /* Output a flag, only if filter allows it. */
@@ -180,11 +180,11 @@ static int show_default(void)
 	const char *s = def;
 
 	if (s) {
-		unsigned char sha1[20];
+		struct object_id oid;
 
 		def = NULL;
-		if (!get_sha1(s, sha1)) {
-			show_rev(NORMAL, sha1, s);
+		if (!get_oid(s, &oid)) {
+			show_rev(NORMAL, &oid, s);
 			return 1;
 		}
 	}
@@ -195,19 +195,19 @@ static int show_reference(const char *refname, const struct object_id *oid, int
 {
 	if (ref_excluded(ref_excludes, refname))
 		return 0;
-	show_rev(NORMAL, oid->hash, refname);
+	show_rev(NORMAL, oid, refname);
 	return 0;
 }
 
 static int anti_reference(const char *refname, const struct object_id *oid, int flag, void *cb_data)
 {
-	show_rev(REVERSED, oid->hash, refname);
+	show_rev(REVERSED, oid, refname);
 	return 0;
 }
 
 static int show_abbrev(const struct object_id *oid, void *cb_data)
 {
-	show_rev(NORMAL, oid->hash, NULL);
+	show_rev(NORMAL, oid, NULL);
 	return 0;
 }
 
@@ -242,8 +242,8 @@ static int show_file(const char *arg, int output_prefix)
 static int try_difference(const char *arg)
 {
 	char *dotdot;
-	unsigned char sha1[20];
-	unsigned char end[20];
+	struct object_id oid;
+	struct object_id end;
 	const char *next;
 	const char *this;
 	int symmetric;
@@ -273,18 +273,18 @@ static int try_difference(const char *arg)
 		return 0;
 	}
 
-	if (!get_sha1_committish(this, sha1) && !get_sha1_committish(next, end)) {
-		show_rev(NORMAL, end, next);
-		show_rev(symmetric ? NORMAL : REVERSED, sha1, this);
+	if (!get_sha1_committish(this, oid.hash) && !get_sha1_committish(next, end.hash)) {
+		show_rev(NORMAL, &end, next);
+		show_rev(symmetric ? NORMAL : REVERSED, &oid, this);
 		if (symmetric) {
 			struct commit_list *exclude;
 			struct commit *a, *b;
-			a = lookup_commit_reference(sha1);
-			b = lookup_commit_reference(end);
+			a = lookup_commit_reference(&oid);
+			b = lookup_commit_reference(&end);
 			exclude = get_merge_bases(a, b);
 			while (exclude) {
 				struct commit *commit = pop_commit(&exclude);
-				show_rev(REVERSED, commit->object.oid.hash, NULL);
+				show_rev(REVERSED, &commit->object.oid, NULL);
 			}
 		}
 		*dotdot = '.';
@@ -297,7 +297,7 @@ static int try_difference(const char *arg)
 static int try_parent_shorthands(const char *arg)
 {
 	char *dotdot;
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct commit *commit;
 	struct commit_list *parents;
 	int parent_number;
@@ -327,12 +327,12 @@ static int try_parent_shorthands(const char *arg)
 		return 0;
 
 	*dotdot = 0;
-	if (get_sha1_committish(arg, sha1)) {
+	if (get_sha1_committish(arg, oid.hash)) {
 		*dotdot = '^';
 		return 0;
 	}
 
-	commit = lookup_commit_reference(sha1);
+	commit = lookup_commit_reference(&oid);
 	if (exclude_parent &&
 	    exclude_parent > commit_list_count(commit->parents)) {
 		*dotdot = '^';
@@ -340,7 +340,7 @@ static int try_parent_shorthands(const char *arg)
 	}
 
 	if (include_rev)
-		show_rev(NORMAL, sha1, arg);
+		show_rev(NORMAL, &oid, arg);
 	for (parents = commit->parents, parent_number = 1;
 	     parents;
 	     parents = parents->next, parent_number++) {
@@ -352,7 +352,7 @@ static int try_parent_shorthands(const char *arg)
 		if (symbolic)
 			name = xstrfmt("%s^%d", arg, parent_number);
 		show_rev(include_parents ? NORMAL : REVERSED,
-			 parents->item->object.oid.hash, name);
+			 &parents->item->object.oid, name);
 		free(name);
 	}
 
@@ -571,7 +571,7 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 	int did_repo_setup = 0;
 	int has_dashdash = 0;
 	int output_prefix = 0;
-	unsigned char sha1[20];
+	struct object_id oid;
 	unsigned int flags = 0;
 	const char *name = NULL;
 	struct object_context unused;
@@ -910,11 +910,11 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 			name++;
 			type = REVERSED;
 		}
-		if (!get_sha1_with_context(name, flags, sha1, &unused)) {
+		if (!get_sha1_with_context(name, flags, oid.hash, &unused)) {
 			if (verify)
 				revs_count++;
 			else
-				show_rev(type, sha1, name);
+				show_rev(type, &oid, name);
 			continue;
 		}
 		if (verify)
@@ -929,7 +929,7 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 	strbuf_release(&buf);
 	if (verify) {
 		if (revs_count == 1) {
-			show_rev(type, sha1, name);
+			show_rev(type, &oid, name);
 			return 0;
 		} else if (revs_count == 0 && show_default())
 			return 0;
diff --git a/builtin/show-branch.c b/builtin/show-branch.c
index 8860f429b0..4a6cc6f490 100644
--- a/builtin/show-branch.c
+++ b/builtin/show-branch.c
@@ -358,7 +358,7 @@ static void sort_ref_range(int bottom, int top)
 static int append_ref(const char *refname, const struct object_id *oid,
 		      int allow_dups)
 {
-	struct commit *commit = lookup_commit_reference_gently(oid->hash, 1);
+	struct commit *commit = lookup_commit_reference_gently(oid, 1);
 	int i;
 
 	if (!commit)
@@ -816,7 +816,7 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 			       MAX_REVS), MAX_REVS);
 		if (get_sha1(ref_name[num_rev], revkey.hash))
 			die(_("'%s' is not a valid ref."), ref_name[num_rev]);
-		commit = lookup_commit_reference(revkey.hash);
+		commit = lookup_commit_reference(&revkey);
 		if (!commit)
 			die(_("cannot find commit %s (%s)"),
 			    ref_name[num_rev], oid_to_hex(&revkey));
diff --git a/builtin/tag.c b/builtin/tag.c
index bdf1e88e93..1f74a56db7 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -66,7 +66,7 @@ static int list_tags(struct ref_filter *filter, struct ref_sorting *sorting, con
 }
 
 typedef int (*each_tag_name_fn)(const char *name, const char *ref,
-				const unsigned char *sha1, const void *cb_data);
+				const struct object_id *oid, const void *cb_data);
 
 static int for_each_tag_name(const char **argv, each_tag_name_fn fn,
 			     const void *cb_data)
@@ -74,17 +74,17 @@ static int for_each_tag_name(const char **argv, each_tag_name_fn fn,
 	const char **p;
 	struct strbuf ref = STRBUF_INIT;
 	int had_error = 0;
-	unsigned char sha1[20];
+	struct object_id oid;
 
 	for (p = argv; *p; p++) {
 		strbuf_reset(&ref);
 		strbuf_addf(&ref, "refs/tags/%s", *p);
-		if (read_ref(ref.buf, sha1)) {
+		if (read_ref(ref.buf, oid.hash)) {
 			error(_("tag '%s' not found."), *p);
 			had_error = 1;
 			continue;
 		}
-		if (fn(*p, ref.buf, sha1, cb_data))
+		if (fn(*p, ref.buf, &oid, cb_data))
 			had_error = 1;
 	}
 	strbuf_release(&ref);
@@ -92,16 +92,16 @@ static int for_each_tag_name(const char **argv, each_tag_name_fn fn,
 }
 
 static int delete_tag(const char *name, const char *ref,
-		      const unsigned char *sha1, const void *cb_data)
+		      const struct object_id *oid, const void *cb_data)
 {
-	if (delete_ref(NULL, ref, sha1, 0))
+	if (delete_ref(NULL, ref, oid->hash, 0))
 		return 1;
-	printf(_("Deleted tag '%s' (was %s)\n"), name, find_unique_abbrev(sha1, DEFAULT_ABBREV));
+	printf(_("Deleted tag '%s' (was %s)\n"), name, find_unique_abbrev(oid->hash, DEFAULT_ABBREV));
 	return 0;
 }
 
 static int verify_tag(const char *name, const char *ref,
-		      const unsigned char *sha1, const void *cb_data)
+		      const struct object_id *oid, const void *cb_data)
 {
 	int flags;
 	const char *fmt_pretty = cb_data;
@@ -110,11 +110,11 @@ static int verify_tag(const char *name, const char *ref,
 	if (fmt_pretty)
 		flags = GPG_VERIFY_OMIT_STATUS;
 
-	if (gpg_verify_tag(sha1, name, flags))
+	if (gpg_verify_tag(oid->hash, name, flags))
 		return -1;
 
 	if (fmt_pretty)
-		pretty_print_ref(name, sha1, fmt_pretty);
+		pretty_print_ref(name, oid->hash, fmt_pretty);
 
 	return 0;
 }
@@ -182,13 +182,13 @@ static int git_tag_config(const char *var, const char *value, void *cb)
 	return git_default_config(var, value, cb);
 }
 
-static void write_tag_body(int fd, const unsigned char *sha1)
+static void write_tag_body(int fd, const struct object_id *oid)
 {
 	unsigned long size;
 	enum object_type type;
 	char *buf, *sp;
 
-	buf = read_sha1_file(sha1, &type, &size);
+	buf = read_sha1_file(oid->hash, &type, &size);
 	if (!buf)
 		return;
 	/* skip header */
@@ -204,11 +204,11 @@ static void write_tag_body(int fd, const unsigned char *sha1)
 	free(buf);
 }
 
-static int build_tag_object(struct strbuf *buf, int sign, unsigned char *result)
+static int build_tag_object(struct strbuf *buf, int sign, struct object_id *result)
 {
 	if (sign && do_sign(buf) < 0)
 		return error(_("unable to sign the tag"));
-	if (write_sha1_file(buf->buf, buf->len, tag_type, result) < 0)
+	if (write_sha1_file(buf->buf, buf->len, tag_type, result->hash) < 0)
 		return error(_("unable to write tag file"));
 	return 0;
 }
@@ -223,15 +223,15 @@ struct create_tag_options {
 	} cleanup_mode;
 };
 
-static void create_tag(const unsigned char *object, const char *tag,
+static void create_tag(const struct object_id *object, const char *tag,
 		       struct strbuf *buf, struct create_tag_options *opt,
-		       unsigned char *prev, unsigned char *result)
+		       struct object_id *prev, struct object_id *result)
 {
 	enum object_type type;
 	struct strbuf header = STRBUF_INIT;
 	char *path = NULL;
 
-	type = sha1_object_info(object, NULL);
+	type = sha1_object_info(object->hash, NULL);
 	if (type <= OBJ_NONE)
 	    die(_("bad object type."));
 
@@ -240,7 +240,7 @@ static void create_tag(const unsigned char *object, const char *tag,
 		    "type %s\n"
 		    "tag %s\n"
 		    "tagger %s\n\n",
-		    sha1_to_hex(object),
+		    oid_to_hex(object),
 		    typename(type),
 		    tag,
 		    git_committer_info(IDENT_STRICT));
@@ -254,7 +254,7 @@ static void create_tag(const unsigned char *object, const char *tag,
 		if (fd < 0)
 			die_errno(_("could not create file '%s'"), path);
 
-		if (!is_null_sha1(prev)) {
+		if (!is_null_oid(prev)) {
 			write_tag_body(fd, prev);
 		} else {
 			struct strbuf buf = STRBUF_INIT;
@@ -296,7 +296,7 @@ static void create_tag(const unsigned char *object, const char *tag,
 	}
 }
 
-static void create_reflog_msg(const unsigned char *sha1, struct strbuf *sb)
+static void create_reflog_msg(const struct object_id *oid, struct strbuf *sb)
 {
 	enum object_type type;
 	struct commit *c;
@@ -310,17 +310,17 @@ static void create_reflog_msg(const unsigned char *sha1, struct strbuf *sb)
 		strbuf_addstr(sb, rla);
 	} else {
 		strbuf_addstr(sb, "tag: tagging ");
-		strbuf_add_unique_abbrev(sb, sha1, DEFAULT_ABBREV);
+		strbuf_add_unique_abbrev(sb, oid->hash, DEFAULT_ABBREV);
 	}
 
 	strbuf_addstr(sb, " (");
-	type = sha1_object_info(sha1, NULL);
+	type = sha1_object_info(oid->hash, NULL);
 	switch (type) {
 	default:
 		strbuf_addstr(sb, "object of unknown type");
 		break;
 	case OBJ_COMMIT:
-		if ((buf = read_sha1_file(sha1, &type, &size)) != NULL) {
+		if ((buf = read_sha1_file(oid->hash, &type, &size)) != NULL) {
 			subject_len = find_commit_subject(buf, &subject_start);
 			strbuf_insert(sb, sb->len, subject_start, subject_len);
 		} else {
@@ -328,7 +328,7 @@ static void create_reflog_msg(const unsigned char *sha1, struct strbuf *sb)
 		}
 		free(buf);
 
-		if ((c = lookup_commit_reference(sha1)) != NULL)
+		if ((c = lookup_commit_reference(oid)) != NULL)
 			strbuf_addf(sb, ", %s", show_date(c->date, 0, DATE_MODE(SHORT)));
 		break;
 	case OBJ_TREE:
@@ -378,7 +378,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 	struct strbuf buf = STRBUF_INIT;
 	struct strbuf ref = STRBUF_INIT;
 	struct strbuf reflog_msg = STRBUF_INIT;
-	unsigned char object[20], prev[20];
+	struct object_id object, prev;
 	const char *object_ref, *tag;
 	struct create_tag_options opt;
 	char *cleanup_arg = NULL;
@@ -528,14 +528,14 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 	if (argc > 2)
 		die(_("too many params"));
 
-	if (get_sha1(object_ref, object))
+	if (get_oid(object_ref, &object))
 		die(_("Failed to resolve '%s' as a valid ref."), object_ref);
 
 	if (strbuf_check_tag_ref(&ref, tag))
 		die(_("'%s' is not a valid tag name."), tag);
 
-	if (read_ref(ref.buf, prev))
-		hashclr(prev);
+	if (read_ref(ref.buf, prev.hash))
+		oidclr(&prev);
 	else if (!force)
 		die(_("tag '%s' already exists"), tag);
 
@@ -550,24 +550,24 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 	else
 		die(_("Invalid cleanup mode %s"), cleanup_arg);
 
-	create_reflog_msg(object, &reflog_msg);
+	create_reflog_msg(&object, &reflog_msg);
 
 	if (create_tag_object) {
 		if (force_sign_annotate && !annotate)
 			opt.sign = 1;
-		create_tag(object, tag, &buf, &opt, prev, object);
+		create_tag(&object, tag, &buf, &opt, &prev, &object);
 	}
 
 	transaction = ref_transaction_begin(&err);
 	if (!transaction ||
-	    ref_transaction_update(transaction, ref.buf, object, prev,
+	    ref_transaction_update(transaction, ref.buf, object.hash, prev.hash,
 				   create_reflog ? REF_FORCE_CREATE_REFLOG : 0,
 				   reflog_msg.buf, &err) ||
 	    ref_transaction_commit(transaction, &err))
 		die("%s", err.buf);
 	ref_transaction_free(transaction);
-	if (force && !is_null_sha1(prev) && hashcmp(prev, object))
-		printf(_("Updated tag '%s' (was %s)\n"), tag, find_unique_abbrev(prev, DEFAULT_ABBREV));
+	if (force && !is_null_oid(&prev) && oidcmp(&prev, &object))
+		printf(_("Updated tag '%s' (was %s)\n"), tag, find_unique_abbrev(prev.hash, DEFAULT_ABBREV));
 
 	strbuf_release(&err);
 	strbuf_release(&buf);
diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 4532aa0831..8bc9997767 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -127,7 +127,7 @@ static void *get_data(unsigned long size)
 }
 
 struct delta_info {
-	unsigned char base_sha1[20];
+	struct object_id base_oid;
 	unsigned nr;
 	off_t base_offset;
 	unsigned long size;
@@ -137,13 +137,13 @@ struct delta_info {
 
 static struct delta_info *delta_list;
 
-static void add_delta_to_list(unsigned nr, unsigned const char *base_sha1,
+static void add_delta_to_list(unsigned nr, const struct object_id *base_oid,
 			      off_t base_offset,
 			      void *delta, unsigned long size)
 {
 	struct delta_info *info = xmalloc(sizeof(*info));
 
-	hashcpy(info->base_sha1, base_sha1);
+	oidcpy(&info->base_oid, base_oid);
 	info->base_offset = base_offset;
 	info->size = size;
 	info->delta = delta;
@@ -154,7 +154,7 @@ static void add_delta_to_list(unsigned nr, unsigned const char *base_sha1,
 
 struct obj_info {
 	off_t offset;
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct object *obj;
 };
 
@@ -170,9 +170,9 @@ static unsigned nr_objects;
  */
 static void write_cached_object(struct object *obj, struct obj_buffer *obj_buf)
 {
-	unsigned char sha1[20];
+	struct object_id oid;
 
-	if (write_sha1_file(obj_buf->buffer, obj_buf->size, typename(obj->type), sha1) < 0)
+	if (write_sha1_file(obj_buf->buffer, obj_buf->size, typename(obj->type), oid.hash) < 0)
 		die("failed to write object %s", oid_to_hex(&obj->oid));
 	obj->flags |= FLAG_WRITTEN;
 }
@@ -237,19 +237,19 @@ static void write_object(unsigned nr, enum object_type type,
 			 void *buf, unsigned long size)
 {
 	if (!strict) {
-		if (write_sha1_file(buf, size, typename(type), obj_list[nr].sha1) < 0)
+		if (write_sha1_file(buf, size, typename(type), obj_list[nr].oid.hash) < 0)
 			die("failed to write object");
 		added_object(nr, type, buf, size);
 		free(buf);
 		obj_list[nr].obj = NULL;
 	} else if (type == OBJ_BLOB) {
 		struct blob *blob;
-		if (write_sha1_file(buf, size, typename(type), obj_list[nr].sha1) < 0)
+		if (write_sha1_file(buf, size, typename(type), obj_list[nr].oid.hash) < 0)
 			die("failed to write object");
 		added_object(nr, type, buf, size);
 		free(buf);
 
-		blob = lookup_blob(obj_list[nr].sha1);
+		blob = lookup_blob(&obj_list[nr].oid);
 		if (blob)
 			blob->object.flags |= FLAG_WRITTEN;
 		else
@@ -258,9 +258,10 @@ static void write_object(unsigned nr, enum object_type type,
 	} else {
 		struct object *obj;
 		int eaten;
-		hash_sha1_file(buf, size, typename(type), obj_list[nr].sha1);
+		hash_sha1_file(buf, size, typename(type), obj_list[nr].oid.hash);
 		added_object(nr, type, buf, size);
-		obj = parse_object_buffer(obj_list[nr].sha1, type, size, buf, &eaten);
+		obj = parse_object_buffer(&obj_list[nr].oid, type, size, buf,
+					  &eaten);
 		if (!obj)
 			die("invalid %s", typename(type));
 		add_object_buffer(obj, buf, size);
@@ -296,7 +297,7 @@ static void added_object(unsigned nr, enum object_type type,
 	struct delta_info *info;
 
 	while ((info = *p) != NULL) {
-		if (!hashcmp(info->base_sha1, obj_list[nr].sha1) ||
+		if (!oidcmp(&info->base_oid, &obj_list[nr].oid) ||
 		    info->base_offset == obj_list[nr].offset) {
 			*p = info->next;
 			p = &delta_list;
@@ -320,12 +321,12 @@ static void unpack_non_delta_entry(enum object_type type, unsigned long size,
 		free(buf);
 }
 
-static int resolve_against_held(unsigned nr, const unsigned char *base,
+static int resolve_against_held(unsigned nr, const struct object_id *base,
 				void *delta_data, unsigned long delta_size)
 {
 	struct object *obj;
 	struct obj_buffer *obj_buffer;
-	obj = lookup_object(base);
+	obj = lookup_object(base->hash);
 	if (!obj)
 		return 0;
 	obj_buffer = lookup_object_buffer(obj);
@@ -341,25 +342,25 @@ static void unpack_delta_entry(enum object_type type, unsigned long delta_size,
 {
 	void *delta_data, *base;
 	unsigned long base_size;
-	unsigned char base_sha1[20];
+	struct object_id base_oid;
 
 	if (type == OBJ_REF_DELTA) {
-		hashcpy(base_sha1, fill(20));
-		use(20);
+		hashcpy(base_oid.hash, fill(GIT_SHA1_RAWSZ));
+		use(GIT_SHA1_RAWSZ);
 		delta_data = get_data(delta_size);
 		if (dry_run || !delta_data) {
 			free(delta_data);
 			return;
 		}
-		if (has_sha1_file(base_sha1))
+		if (has_object_file(&base_oid))
 			; /* Ok we have this one */
-		else if (resolve_against_held(nr, base_sha1,
+		else if (resolve_against_held(nr, &base_oid,
 					      delta_data, delta_size))
 			return; /* we are done */
 		else {
 			/* cannot resolve yet --- queue it */
-			hashclr(obj_list[nr].sha1);
-			add_delta_to_list(nr, base_sha1, 0, delta_data, delta_size);
+			oidclr(&obj_list[nr].oid);
+			add_delta_to_list(nr, &base_oid, 0, delta_data, delta_size);
 			return;
 		}
 	} else {
@@ -399,8 +400,8 @@ static void unpack_delta_entry(enum object_type type, unsigned long delta_size,
 			} else if (base_offset > obj_list[mid].offset) {
 				lo = mid + 1;
 			} else {
-				hashcpy(base_sha1, obj_list[mid].sha1);
-				base_found = !is_null_sha1(base_sha1);
+				oidcpy(&base_oid, &obj_list[mid].oid);
+				base_found = !is_null_oid(&base_oid);
 				break;
 			}
 		}
@@ -409,19 +410,19 @@ static void unpack_delta_entry(enum object_type type, unsigned long delta_size,
 			 * The delta base object is itself a delta that
 			 * has not been resolved yet.
 			 */
-			hashclr(obj_list[nr].sha1);
-			add_delta_to_list(nr, null_sha1, base_offset, delta_data, delta_size);
+			oidclr(&obj_list[nr].oid);
+			add_delta_to_list(nr, &null_oid, base_offset, delta_data, delta_size);
 			return;
 		}
 	}
 
-	if (resolve_against_held(nr, base_sha1, delta_data, delta_size))
+	if (resolve_against_held(nr, &base_oid, delta_data, delta_size))
 		return;
 
-	base = read_sha1_file(base_sha1, &type, &base_size);
+	base = read_sha1_file(base_oid.hash, &type, &base_size);
 	if (!base) {
 		error("failed to read delta-pack base object %s",
-		      sha1_to_hex(base_sha1));
+		      oid_to_hex(&base_oid));
 		if (!recover)
 			exit(1);
 		has_errors = 1;
@@ -505,7 +506,7 @@ static void unpack_all(void)
 int cmd_unpack_objects(int argc, const char **argv, const char *prefix)
 {
 	int i;
-	unsigned char sha1[20];
+	struct object_id oid;
 
 	check_replace_refs = 0;
 
@@ -566,12 +567,12 @@ int cmd_unpack_objects(int argc, const char **argv, const char *prefix)
 	git_SHA1_Init(&ctx);
 	unpack_all();
 	git_SHA1_Update(&ctx, buffer, offset);
-	git_SHA1_Final(sha1, &ctx);
+	git_SHA1_Final(oid.hash, &ctx);
 	if (strict)
 		write_rest();
-	if (hashcmp(fill(20), sha1))
+	if (hashcmp(fill(GIT_SHA1_RAWSZ), oid.hash))
 		die("final sha1 did not match");
-	use(20);
+	use(GIT_SHA1_RAWSZ);
 
 	/* Write the last part of the buffer to stdout */
 	while (len) {
diff --git a/builtin/verify-commit.c b/builtin/verify-commit.c
index 38bedf8f9f..05b734e6d1 100644
--- a/builtin/verify-commit.c
+++ b/builtin/verify-commit.c
@@ -18,14 +18,14 @@ static const char * const verify_commit_usage[] = {
 		NULL
 };
 
-static int run_gpg_verify(const unsigned char *sha1, const char *buf, unsigned long size, unsigned flags)
+static int run_gpg_verify(const struct object_id *oid, const char *buf, unsigned long size, unsigned flags)
 {
 	struct signature_check signature_check;
 	int ret;
 
 	memset(&signature_check, 0, sizeof(signature_check));
 
-	ret = check_commit_signature(lookup_commit(sha1), &signature_check);
+	ret = check_commit_signature(lookup_commit(oid), &signature_check);
 	print_signature_buffer(&signature_check, flags);
 
 	signature_check_clear(&signature_check);
@@ -35,22 +35,22 @@ static int run_gpg_verify(const unsigned char *sha1, const char *buf, unsigned l
 static int verify_commit(const char *name, unsigned flags)
 {
 	enum object_type type;
-	unsigned char sha1[20];
+	struct object_id oid;
 	char *buf;
 	unsigned long size;
 	int ret;
 
-	if (get_sha1(name, sha1))
+	if (get_oid(name, &oid))
 		return error("commit '%s' not found.", name);
 
-	buf = read_sha1_file(sha1, &type, &size);
+	buf = read_sha1_file(oid.hash, &type, &size);
 	if (!buf)
 		return error("%s: unable to read file.", name);
 	if (type != OBJ_COMMIT)
 		return error("%s: cannot verify a non-commit object of type %s.",
 				name, typename(type));
 
-	ret = run_gpg_verify(sha1, buf, size, flags);
+	ret = run_gpg_verify(&oid, buf, size, flags);
 
 	free(buf);
 	return ret;
diff --git a/bulk-checkin.c b/bulk-checkin.c
index ddb6070c4c..5be7ce5c73 100644
--- a/bulk-checkin.c
+++ b/bulk-checkin.c
@@ -69,7 +69,7 @@ static int already_written(struct bulk_checkin_state *state, unsigned char sha1[
 
 	/* Might want to keep the list sorted */
 	for (i = 0; i < state->nr_written; i++)
-		if (!hashcmp(state->written[i]->sha1, sha1))
+		if (!hashcmp(state->written[i]->oid.hash, sha1))
 			return 1;
 
 	/* This is a new object we need to keep */
@@ -242,7 +242,7 @@ static int deflate_to_pack(struct bulk_checkin_state *state,
 		state->offset = checkpoint.offset;
 		free(idx);
 	} else {
-		hashcpy(idx->sha1, result_sha1);
+		hashcpy(idx->oid.hash, result_sha1);
 		ALLOC_GROW(state->written,
 			   state->nr_written + 1,
 			   state->alloc_written);
diff --git a/bundle.c b/bundle.c
index 05e014fc5a..d15db03c84 100644
--- a/bundle.c
+++ b/bundle.c
@@ -12,11 +12,11 @@
 
 static const char bundle_signature[] = "# v2 git bundle\n";
 
-static void add_to_ref_list(const unsigned char *sha1, const char *name,
+static void add_to_ref_list(const struct object_id *oid, const char *name,
 		struct ref_list *list)
 {
 	ALLOC_GROW(list->list, list->nr + 1, list->alloc);
-	hashcpy(list->list[list->nr].sha1, sha1);
+	oidcpy(&list->list[list->nr].oid, oid);
 	list->list[list->nr].name = xstrdup(name);
 	list->nr++;
 }
@@ -40,8 +40,9 @@ static int parse_bundle_header(int fd, struct bundle_header *header,
 	/* The bundle header ends with an empty line */
 	while (!strbuf_getwholeline_fd(&buf, fd, '\n') &&
 	       buf.len && buf.buf[0] != '\n') {
-		unsigned char sha1[20];
+		struct object_id oid;
 		int is_prereq = 0;
+		const char *p;
 
 		if (*buf.buf == '-') {
 			is_prereq = 1;
@@ -54,9 +55,9 @@ static int parse_bundle_header(int fd, struct bundle_header *header,
 		 * Prerequisites have object name that is optionally
 		 * followed by SP and subject line.
 		 */
-		if (get_sha1_hex(buf.buf, sha1) ||
-		    (buf.len > 40 && !isspace(buf.buf[40])) ||
-		    (!is_prereq && buf.len <= 40)) {
+		if (parse_oid_hex(buf.buf, &oid, &p) ||
+		    (*p && !isspace(*p)) ||
+		    (!is_prereq && !*p)) {
 			if (report_path)
 				error(_("unrecognized header: %s%s (%d)"),
 				      (is_prereq ? "-" : ""), buf.buf, (int)buf.len);
@@ -64,9 +65,9 @@ static int parse_bundle_header(int fd, struct bundle_header *header,
 			break;
 		} else {
 			if (is_prereq)
-				add_to_ref_list(sha1, "", &header->prerequisites);
+				add_to_ref_list(&oid, "", &header->prerequisites);
 			else
-				add_to_ref_list(sha1, buf.buf + 41, &header->references);
+				add_to_ref_list(&oid, p + 1, &header->references);
 		}
 	}
 
@@ -115,7 +116,7 @@ static int list_refs(struct ref_list *r, int argc, const char **argv)
 			if (j == argc)
 				continue;
 		}
-		printf("%s %s\n", sha1_to_hex(r->list[i].sha1),
+		printf("%s %s\n", oid_to_hex(&r->list[i].oid),
 				r->list[i].name);
 	}
 	return 0;
@@ -141,7 +142,7 @@ int verify_bundle(struct bundle_header *header, int verbose)
 	init_revisions(&revs, NULL);
 	for (i = 0; i < p->nr; i++) {
 		struct ref_list_entry *e = p->list + i;
-		struct object *o = parse_object(e->sha1);
+		struct object *o = parse_object(&e->oid);
 		if (o) {
 			o->flags |= PREREQ_MARK;
 			add_pending_object(&revs, o, e->name);
@@ -149,7 +150,7 @@ int verify_bundle(struct bundle_header *header, int verbose)
 		}
 		if (++ret == 1)
 			error("%s", message);
-		error("%s %s", sha1_to_hex(e->sha1), e->name);
+		error("%s %s", oid_to_hex(&e->oid), e->name);
 	}
 	if (revs.pending.nr != p->nr)
 		return ret;
@@ -285,16 +286,18 @@ static int compute_and_write_prerequisites(int bundle_fd,
 		return -1;
 	rls_fout = xfdopen(rls.out, "r");
 	while (strbuf_getwholeline(&buf, rls_fout, '\n') != EOF) {
-		unsigned char sha1[20];
+		struct object_id oid;
 		if (buf.len > 0 && buf.buf[0] == '-') {
 			write_or_die(bundle_fd, buf.buf, buf.len);
-			if (!get_sha1_hex(buf.buf + 1, sha1)) {
-				struct object *object = parse_object_or_die(sha1, buf.buf);
+			if (!get_oid_hex(buf.buf + 1, &oid)) {
+				struct object *object = parse_object_or_die(&oid,
+									    buf.buf);
 				object->flags |= UNINTERESTING;
 				add_pending_object(revs, object, buf.buf);
 			}
-		} else if (!get_sha1_hex(buf.buf, sha1)) {
-			struct object *object = parse_object_or_die(sha1, buf.buf);
+		} else if (!get_oid_hex(buf.buf, &oid)) {
+			struct object *object = parse_object_or_die(&oid,
+								    buf.buf);
 			object->flags |= SHOWN;
 		}
 	}
@@ -366,7 +369,7 @@ static int write_bundle_refs(int bundle_fd, struct rev_info *revs)
 			 * in terms of a tag (e.g. v2.0 from the range
 			 * "v1.0..v2.0")?
 			 */
-			struct commit *one = lookup_commit_reference(oid.hash);
+			struct commit *one = lookup_commit_reference(&oid);
 			struct object *obj;
 
 			if (e->item == &(one->object)) {
@@ -378,7 +381,7 @@ static int write_bundle_refs(int bundle_fd, struct rev_info *revs)
 				 * end up triggering "empty bundle"
 				 * error.
 				 */
-				obj = parse_object_or_die(oid.hash, e->name);
+				obj = parse_object_or_die(&oid, e->name);
 				obj->flags |= SHOWN;
 				add_pending_object(revs, obj, e->name);
 			}
diff --git a/bundle.h b/bundle.h
index 1584e4d821..e9a4cb6a74 100644
--- a/bundle.h
+++ b/bundle.h
@@ -1,10 +1,12 @@
 #ifndef BUNDLE_H
 #define BUNDLE_H
 
+#include "cache.h"
+
 struct ref_list {
 	unsigned int nr, alloc;
 	struct ref_list_entry {
-		unsigned char sha1[20];
+		struct object_id oid;
 		char *name;
 	} *list;
 };
diff --git a/cache-tree.c b/cache-tree.c
index 34baa6d85a..ec23d8c03d 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -225,7 +225,7 @@ int cache_tree_fully_valid(struct cache_tree *it)
 	int i;
 	if (!it)
 		return 0;
-	if (it->entry_count < 0 || !has_sha1_file(it->sha1))
+	if (it->entry_count < 0 || !has_sha1_file(it->oid.hash))
 		return 0;
 	for (i = 0; i < it->subtree_nr; i++) {
 		if (!cache_tree_fully_valid(it->down[i]->cache_tree))
@@ -253,7 +253,7 @@ static int update_one(struct cache_tree *it,
 
 	*skip_count = 0;
 
-	if (0 <= it->entry_count && has_sha1_file(it->sha1))
+	if (0 <= it->entry_count && has_sha1_file(it->oid.hash))
 		return it->entry_count;
 
 	/*
@@ -340,7 +340,7 @@ static int update_one(struct cache_tree *it,
 				die("cache-tree.c: '%.*s' in '%s' not found",
 				    entlen, path + baselen, path);
 			i += sub->count;
-			sha1 = sub->cache_tree->sha1;
+			sha1 = sub->cache_tree->oid.hash;
 			mode = S_IFDIR;
 			contains_ita = sub->cache_tree->entry_count < 0;
 			if (contains_ita) {
@@ -404,12 +404,13 @@ static int update_one(struct cache_tree *it,
 		unsigned char sha1[20];
 		hash_sha1_file(buffer.buf, buffer.len, tree_type, sha1);
 		if (has_sha1_file(sha1))
-			hashcpy(it->sha1, sha1);
+			hashcpy(it->oid.hash, sha1);
 		else
 			to_invalidate = 1;
 	} else if (dryrun)
-		hash_sha1_file(buffer.buf, buffer.len, tree_type, it->sha1);
-	else if (write_sha1_file(buffer.buf, buffer.len, tree_type, it->sha1)) {
+		hash_sha1_file(buffer.buf, buffer.len, tree_type,
+			       it->oid.hash);
+	else if (write_sha1_file(buffer.buf, buffer.len, tree_type, it->oid.hash)) {
 		strbuf_release(&buffer);
 		return -1;
 	}
@@ -419,7 +420,7 @@ static int update_one(struct cache_tree *it,
 #if DEBUG
 	fprintf(stderr, "cache-tree update-one (%d ent, %d subtree) %s\n",
 		it->entry_count, it->subtree_nr,
-		sha1_to_hex(it->sha1));
+		oid_to_hex(&it->oid));
 #endif
 	return i;
 }
@@ -459,14 +460,14 @@ static void write_one(struct strbuf *buffer, struct cache_tree *it,
 	if (0 <= it->entry_count)
 		fprintf(stderr, "cache-tree <%.*s> (%d ent, %d subtree) %s\n",
 			pathlen, path, it->entry_count, it->subtree_nr,
-			sha1_to_hex(it->sha1));
+			oid_to_hex(&it->oid));
 	else
 		fprintf(stderr, "cache-tree <%.*s> (%d subtree) invalid\n",
 			pathlen, path, it->subtree_nr);
 #endif
 
 	if (0 <= it->entry_count) {
-		strbuf_add(buffer, it->sha1, 20);
+		strbuf_add(buffer, it->oid.hash, 20);
 	}
 	for (i = 0; i < it->subtree_nr; i++) {
 		struct cache_tree_sub *down = it->down[i];
@@ -523,7 +524,7 @@ static struct cache_tree *read_one(const char **buffer, unsigned long *size_p)
 	if (0 <= it->entry_count) {
 		if (size < 20)
 			goto free_return;
-		hashcpy(it->sha1, (const unsigned char*)buf);
+		hashcpy(it->oid.hash, (const unsigned char*)buf);
 		buf += 20;
 		size -= 20;
 	}
@@ -532,7 +533,7 @@ static struct cache_tree *read_one(const char **buffer, unsigned long *size_p)
 	if (0 <= it->entry_count)
 		fprintf(stderr, "cache-tree <%s> (%d ent, %d subtree) %s\n",
 			*buffer, it->entry_count, subtree_nr,
-			sha1_to_hex(it->sha1));
+			oid_to_hex(&it->oid));
 	else
 		fprintf(stderr, "cache-tree <%s> (%d subtrees) invalid\n",
 			*buffer, subtree_nr);
@@ -643,10 +644,10 @@ int write_index_as_tree(unsigned char *sha1, struct index_state *index_state, co
 		subtree = cache_tree_find(index_state->cache_tree, prefix);
 		if (!subtree)
 			return WRITE_TREE_PREFIX_ERROR;
-		hashcpy(sha1, subtree->sha1);
+		hashcpy(sha1, subtree->oid.hash);
 	}
 	else
-		hashcpy(sha1, index_state->cache_tree->sha1);
+		hashcpy(sha1, index_state->cache_tree->oid.hash);
 
 	if (0 <= newfd)
 		rollback_lock_file(lock_file);
@@ -665,7 +666,7 @@ static void prime_cache_tree_rec(struct cache_tree *it, struct tree *tree)
 	struct name_entry entry;
 	int cnt;
 
-	hashcpy(it->sha1, tree->object.oid.hash);
+	oidcpy(&it->oid, &tree->object.oid);
 	init_tree_desc(&desc, tree->buffer, tree->size);
 	cnt = 0;
 	while (tree_entry(&desc, &entry)) {
@@ -673,7 +674,7 @@ static void prime_cache_tree_rec(struct cache_tree *it, struct tree *tree)
 			cnt++;
 		else {
 			struct cache_tree_sub *sub;
-			struct tree *subtree = lookup_tree(entry.oid->hash);
+			struct tree *subtree = lookup_tree(entry.oid);
 			if (!subtree->object.parsed)
 				parse_tree(subtree);
 			sub = cache_tree_sub(it, entry.path);
@@ -720,7 +721,7 @@ int cache_tree_matches_traversal(struct cache_tree *root,
 
 	it = find_cache_tree_from_traversal(root, info);
 	it = cache_tree_find(it, ent->path);
-	if (it && it->entry_count > 0 && !hashcmp(ent->oid->hash, it->sha1))
+	if (it && it->entry_count > 0 && !oidcmp(ent->oid, &it->oid))
 		return it->entry_count;
 	return 0;
 }
diff --git a/cache-tree.h b/cache-tree.h
index 41c574663a..f7b9cab7ee 100644
--- a/cache-tree.h
+++ b/cache-tree.h
@@ -1,6 +1,7 @@
 #ifndef CACHE_TREE_H
 #define CACHE_TREE_H
 
+#include "cache.h"
 #include "tree.h"
 #include "tree-walk.h"
 
@@ -15,7 +16,7 @@ struct cache_tree_sub {
 
 struct cache_tree {
 	int entry_count; /* negative means "invalid" */
-	unsigned char sha1[20];
+	struct object_id oid;
 	int subtree_nr;
 	int subtree_alloc;
 	struct cache_tree_sub **down;
diff --git a/cache.h b/cache.h
index 188811920c..d1f2c5c088 100644
--- a/cache.h
+++ b/cache.h
@@ -2198,8 +2198,8 @@ struct commit_list;
 int try_merge_command(const char *strategy, size_t xopts_nr,
 		const char **xopts, struct commit_list *common,
 		const char *head_arg, struct commit_list *remotes);
-int checkout_fast_forward(const unsigned char *from,
-			  const unsigned char *to,
+int checkout_fast_forward(const struct object_id *from,
+			  const struct object_id *to,
 			  int overwrite_ignore);
 
 
diff --git a/commit.c b/commit.c
index 99a62b90ee..2d39dadee1 100644
--- a/commit.c
+++ b/commit.c
@@ -18,38 +18,38 @@ int save_commit_buffer = 1;
 
 const char *commit_type = "commit";
 
-struct commit *lookup_commit_reference_gently(const unsigned char *sha1,
+struct commit *lookup_commit_reference_gently(const struct object_id *oid,
 					      int quiet)
 {
-	struct object *obj = deref_tag(parse_object(sha1), NULL, 0);
+	struct object *obj = deref_tag(parse_object(oid), NULL, 0);
 
 	if (!obj)
 		return NULL;
 	return object_as_type(obj, OBJ_COMMIT, quiet);
 }
 
-struct commit *lookup_commit_reference(const unsigned char *sha1)
+struct commit *lookup_commit_reference(const struct object_id *oid)
 {
-	return lookup_commit_reference_gently(sha1, 0);
+	return lookup_commit_reference_gently(oid, 0);
 }
 
-struct commit *lookup_commit_or_die(const unsigned char *sha1, const char *ref_name)
+struct commit *lookup_commit_or_die(const struct object_id *oid, const char *ref_name)
 {
-	struct commit *c = lookup_commit_reference(sha1);
+	struct commit *c = lookup_commit_reference(oid);
 	if (!c)
 		die(_("could not parse %s"), ref_name);
-	if (hashcmp(sha1, c->object.oid.hash)) {
+	if (oidcmp(oid, &c->object.oid)) {
 		warning(_("%s %s is not a commit!"),
-			ref_name, sha1_to_hex(sha1));
+			ref_name, oid_to_hex(oid));
 	}
 	return c;
 }
 
-struct commit *lookup_commit(const unsigned char *sha1)
+struct commit *lookup_commit(const struct object_id *oid)
 {
-	struct object *obj = lookup_object(sha1);
+	struct object *obj = lookup_object(oid->hash);
 	if (!obj)
-		return create_object(sha1, alloc_commit_node());
+		return create_object(oid->hash, alloc_commit_node());
 	return object_as_type(obj, OBJ_COMMIT, 0);
 }
 
@@ -60,7 +60,7 @@ struct commit *lookup_commit_reference_by_name(const char *name)
 
 	if (get_sha1_committish(name, oid.hash))
 		return NULL;
-	commit = lookup_commit_reference(oid.hash);
+	commit = lookup_commit_reference(&oid);
 	if (parse_commit(commit))
 		return NULL;
 	return commit;
@@ -216,9 +216,9 @@ int for_each_commit_graft(each_commit_graft_fn fn, void *cb_data)
 	return ret;
 }
 
-int unregister_shallow(const unsigned char *sha1)
+int unregister_shallow(const struct object_id *oid)
 {
-	int pos = commit_graft_pos(sha1);
+	int pos = commit_graft_pos(oid->hash);
 	if (pos < 0)
 		return -1;
 	if (pos + 1 < commit_graft_nr)
@@ -331,7 +331,7 @@ int parse_commit_buffer(struct commit *item, const void *buffer, unsigned long s
 	if (get_sha1_hex(bufptr + 5, parent.hash) < 0)
 		return error("bad tree pointer in commit %s",
 			     oid_to_hex(&item->object.oid));
-	item->tree = lookup_tree(parent.hash);
+	item->tree = lookup_tree(&parent);
 	bufptr += tree_entry_len + 1; /* "tree " + "hex sha1" + "\n" */
 	pptr = &item->parents;
 
@@ -350,7 +350,7 @@ int parse_commit_buffer(struct commit *item, const void *buffer, unsigned long s
 		 */
 		if (graft && (graft->nr_parent < 0 || grafts_replace_parents))
 			continue;
-		new_parent = lookup_commit(parent.hash);
+		new_parent = lookup_commit(&parent);
 		if (new_parent)
 			pptr = &commit_list_insert(new_parent, pptr)->next;
 	}
@@ -358,7 +358,7 @@ int parse_commit_buffer(struct commit *item, const void *buffer, unsigned long s
 		int i;
 		struct commit *new_parent;
 		for (i = 0; i < graft->nr_parent; i++) {
-			new_parent = lookup_commit(graft->parent[i].hash);
+			new_parent = lookup_commit(&graft->parent[i]);
 			if (!new_parent)
 				continue;
 			pptr = &commit_list_insert(new_parent, pptr)->next;
@@ -562,7 +562,7 @@ void clear_commit_marks_for_object_array(struct object_array *a, unsigned mark)
 
 	for (i = 0; i < a->nr; i++) {
 		object = a->objects[i].item;
-		commit = lookup_commit_reference_gently(object->oid.hash, 1);
+		commit = lookup_commit_reference_gently(&object->oid, 1);
 		if (commit)
 			clear_commit_marks(commit, mark);
 	}
@@ -1589,7 +1589,7 @@ struct commit *get_merge_parent(const char *name)
 	struct object_id oid;
 	if (get_sha1(name, oid.hash))
 		return NULL;
-	obj = parse_object(oid.hash);
+	obj = parse_object(&oid);
 	commit = (struct commit *)peel_to_type(name, 0, obj, OBJ_COMMIT);
 	if (commit && !commit->util)
 		set_merge_remote_desc(commit, name, obj);
diff --git a/commit.h b/commit.h
index c9d887b5e5..4127c298cb 100644
--- a/commit.h
+++ b/commit.h
@@ -45,18 +45,18 @@ enum decoration_type {
 void add_name_decoration(enum decoration_type type, const char *name, struct object *obj);
 const struct name_decoration *get_name_decoration(const struct object *obj);
 
-struct commit *lookup_commit(const unsigned char *sha1);
-struct commit *lookup_commit_reference(const unsigned char *sha1);
-struct commit *lookup_commit_reference_gently(const unsigned char *sha1,
+struct commit *lookup_commit(const struct object_id *oid);
+struct commit *lookup_commit_reference(const struct object_id *oid);
+struct commit *lookup_commit_reference_gently(const struct object_id *oid,
 					      int quiet);
 struct commit *lookup_commit_reference_by_name(const char *name);
 
 /*
- * Look up object named by "sha1", dereference tag as necessary,
- * get a commit and return it. If "sha1" does not dereference to
+ * Look up object named by "oid", dereference tag as necessary,
+ * get a commit and return it. If "oid" does not dereference to
  * a commit, use ref_name to report an error and die.
  */
-struct commit *lookup_commit_or_die(const unsigned char *sha1, const char *ref_name);
+struct commit *lookup_commit_or_die(const struct object_id *oid, const char *ref_name);
 
 int parse_commit_buffer(struct commit *item, const void *buffer, unsigned long size);
 int parse_commit_gently(struct commit *item, int quiet_on_missing);
@@ -263,8 +263,8 @@ extern struct commit_list *get_merge_bases_many_dirty(struct commit *one, int n,
 
 struct oid_array;
 struct ref;
-extern int register_shallow(const unsigned char *sha1);
-extern int unregister_shallow(const unsigned char *sha1);
+extern int register_shallow(const struct object_id *oid);
+extern int unregister_shallow(const struct object_id *oid);
 extern int for_each_commit_graft(each_commit_graft_fn, void *);
 extern int is_repository_shallow(void);
 extern struct commit_list *get_shallow_commits(struct object_array *heads,
diff --git a/diff-lib.c b/diff-lib.c
index 52447466b5..2982bf055a 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -478,7 +478,7 @@ static int oneway_diff(const struct cache_entry * const *src,
 }
 
 static int diff_cache(struct rev_info *revs,
-		      const unsigned char *tree_sha1,
+		      const struct object_id *tree_oid,
 		      const char *tree_name,
 		      int cached)
 {
@@ -486,10 +486,10 @@ static int diff_cache(struct rev_info *revs,
 	struct tree_desc t;
 	struct unpack_trees_options opts;
 
-	tree = parse_tree_indirect(tree_sha1);
+	tree = parse_tree_indirect(tree_oid);
 	if (!tree)
 		return error("bad tree object %s",
-			     tree_name ? tree_name : sha1_to_hex(tree_sha1));
+			     tree_name ? tree_name : oid_to_hex(tree_oid));
 	memset(&opts, 0, sizeof(opts));
 	opts.head_idx = 1;
 	opts.index_only = cached;
@@ -512,7 +512,7 @@ int run_diff_index(struct rev_info *revs, int cached)
 	struct object_array_entry *ent;
 
 	ent = revs->pending.objects;
-	if (diff_cache(revs, ent->item->oid.hash, ent->name, cached))
+	if (diff_cache(revs, &ent->item->oid, ent->name, cached))
 		exit(128);
 
 	diff_set_mnemonic_prefix(&revs->diffopt, "c/", cached ? "i/" : "w/");
@@ -522,7 +522,7 @@ int run_diff_index(struct rev_info *revs, int cached)
 	return 0;
 }
 
-int do_diff_cache(const unsigned char *tree_sha1, struct diff_options *opt)
+int do_diff_cache(const struct object_id *tree_oid, struct diff_options *opt)
 {
 	struct rev_info revs;
 
@@ -530,7 +530,7 @@ int do_diff_cache(const unsigned char *tree_sha1, struct diff_options *opt)
 	copy_pathspec(&revs.prune_data, &opt->pathspec);
 	revs.diffopt = *opt;
 
-	if (diff_cache(&revs, tree_sha1, NULL, 1))
+	if (diff_cache(&revs, tree_oid, NULL, 1))
 		exit(128);
 	return 0;
 }
diff --git a/diff.c b/diff.c
index 74283d9001..f3546536b4 100644
--- a/diff.c
+++ b/diff.c
@@ -5244,7 +5244,7 @@ size_t fill_textconv(struct userdiff_driver *driver,
 
 	if (driver->textconv_cache && df->oid_valid) {
 		*outbuf = notes_cache_get(driver->textconv_cache,
-					  df->oid.hash,
+					  &df->oid,
 					  &size);
 		if (*outbuf)
 			return size;
@@ -5256,7 +5256,7 @@ size_t fill_textconv(struct userdiff_driver *driver,
 
 	if (driver->textconv_cache && df->oid_valid) {
 		/* ignore errors, as we might be in a readonly repository */
-		notes_cache_put(driver->textconv_cache, df->oid.hash, *outbuf,
+		notes_cache_put(driver->textconv_cache, &df->oid, *outbuf,
 				size);
 		/*
 		 * we could save up changes and flush them all at the end,
diff --git a/diff.h b/diff.h
index 5be1ee77a7..d75e6d15e2 100644
--- a/diff.h
+++ b/diff.h
@@ -354,7 +354,7 @@ extern const char *diff_aligned_abbrev(const struct object_id *sha1, int);
 extern int run_diff_files(struct rev_info *revs, unsigned int option);
 extern int run_diff_index(struct rev_info *revs, int cached);
 
-extern int do_diff_cache(const unsigned char *, struct diff_options *);
+extern int do_diff_cache(const struct object_id *, struct diff_options *);
 extern int diff_flush_patch_id(struct diff_options *, unsigned char *, int);
 
 extern int diff_result_code(struct diff_options *, int);
diff --git a/fast-import.c b/fast-import.c
index cf58f875b8..e69d219682 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -226,7 +226,7 @@ struct tree_entry {
 	struct atom_str *name;
 	struct tree_entry_ms {
 		uint16_t mode;
-		unsigned char sha1[20];
+		struct object_id oid;
 	} versions[2];
 };
 
@@ -252,19 +252,19 @@ struct branch {
 	unsigned active : 1;
 	unsigned delete : 1;
 	unsigned pack_id : PACK_ID_BITS;
-	unsigned char sha1[20];
+	struct object_id oid;
 };
 
 struct tag {
 	struct tag *next_tag;
 	const char *name;
 	unsigned int pack_id;
-	unsigned char sha1[20];
+	struct object_id oid;
 };
 
 struct hash_list {
 	struct hash_list *next;
-	unsigned char sha1[20];
+	struct object_id oid;
 };
 
 typedef enum {
@@ -386,13 +386,15 @@ static void write_branch_report(FILE *rpt, struct branch *b)
 		fputs(" active", rpt);
 	if (b->branch_tree.tree)
 		fputs(" loaded", rpt);
-	if (is_null_sha1(b->branch_tree.versions[1].sha1))
+	if (is_null_oid(&b->branch_tree.versions[1].oid))
 		fputs(" dirty", rpt);
 	fputc('\n', rpt);
 
-	fprintf(rpt, "  tip commit  : %s\n", sha1_to_hex(b->sha1));
-	fprintf(rpt, "  old tree    : %s\n", sha1_to_hex(b->branch_tree.versions[0].sha1));
-	fprintf(rpt, "  cur tree    : %s\n", sha1_to_hex(b->branch_tree.versions[1].sha1));
+	fprintf(rpt, "  tip commit  : %s\n", oid_to_hex(&b->oid));
+	fprintf(rpt, "  old tree    : %s\n",
+		oid_to_hex(&b->branch_tree.versions[0].oid));
+	fprintf(rpt, "  cur tree    : %s\n",
+		oid_to_hex(&b->branch_tree.versions[1].oid));
 	fprintf(rpt, "  commit clock: %" PRIuMAX "\n", b->last_commit);
 
 	fputs("  last pack   : ", rpt);
@@ -470,7 +472,7 @@ static void write_crash_report(const char *err)
 		fputs("Annotated Tags\n", rpt);
 		fputs("--------------\n", rpt);
 		for (tg = first_tag; tg; tg = tg->next_tag) {
-			fputs(sha1_to_hex(tg->sha1), rpt);
+			fputs(oid_to_hex(&tg->oid), rpt);
 			fputc(' ', rpt);
 			fputs(tg->name, rpt);
 			fputc('\n', rpt);
@@ -555,7 +557,7 @@ static void alloc_objects(unsigned int cnt)
 	alloc_count += cnt;
 }
 
-static struct object_entry *new_object(unsigned char *sha1)
+static struct object_entry *new_object(struct object_id *oid)
 {
 	struct object_entry *e;
 
@@ -563,32 +565,32 @@ static struct object_entry *new_object(unsigned char *sha1)
 		alloc_objects(object_entry_alloc);
 
 	e = blocks->next_free++;
-	hashcpy(e->idx.sha1, sha1);
+	oidcpy(&e->idx.oid, oid);
 	return e;
 }
 
-static struct object_entry *find_object(unsigned char *sha1)
+static struct object_entry *find_object(struct object_id *oid)
 {
-	unsigned int h = sha1[0] << 8 | sha1[1];
+	unsigned int h = oid->hash[0] << 8 | oid->hash[1];
 	struct object_entry *e;
 	for (e = object_table[h]; e; e = e->next)
-		if (!hashcmp(sha1, e->idx.sha1))
+		if (!oidcmp(oid, &e->idx.oid))
 			return e;
 	return NULL;
 }
 
-static struct object_entry *insert_object(unsigned char *sha1)
+static struct object_entry *insert_object(struct object_id *oid)
 {
-	unsigned int h = sha1[0] << 8 | sha1[1];
+	unsigned int h = oid->hash[0] << 8 | oid->hash[1];
 	struct object_entry *e = object_table[h];
 
 	while (e) {
-		if (!hashcmp(sha1, e->idx.sha1))
+		if (!oidcmp(oid, &e->idx.oid))
 			return e;
 		e = e->next;
 	}
 
-	e = new_object(sha1);
+	e = new_object(oid);
 	e->next = object_table[h];
 	e->idx.offset = 0;
 	object_table[h] = e;
@@ -876,7 +878,7 @@ static struct tree_content *dup_tree_content(struct tree_content *s)
 		a = s->entries[i];
 		b = new_tree_entry();
 		memcpy(b, a, sizeof(*a));
-		if (a->tree && is_null_sha1(b->versions[1].sha1))
+		if (a->tree && is_null_oid(&b->versions[1].oid))
 			b->tree = dup_tree_content(a->tree);
 		else
 			b->tree = NULL;
@@ -1005,17 +1007,17 @@ static void end_packfile(void)
 	clear_delta_base_cache();
 	if (object_count) {
 		struct packed_git *new_p;
-		unsigned char cur_pack_sha1[20];
+		struct object_id cur_pack_oid;
 		char *idx_name;
 		int i;
 		struct branch *b;
 		struct tag *t;
 
 		close_pack_windows(pack_data);
-		sha1close(pack_file, cur_pack_sha1, 0);
+		sha1close(pack_file, cur_pack_oid.hash, 0);
 		fixup_pack_header_footer(pack_data->pack_fd, pack_data->sha1,
 				    pack_data->pack_name, object_count,
-				    cur_pack_sha1, pack_size);
+				    cur_pack_oid.hash, pack_size);
 
 		if (object_count <= unpack_limit) {
 			if (!loosen_small_pack(pack_data)) {
@@ -1041,12 +1043,14 @@ static void end_packfile(void)
 			for (i = 0; i < branch_table_sz; i++) {
 				for (b = branch_table[i]; b; b = b->table_next_branch) {
 					if (b->pack_id == pack_id)
-						fprintf(pack_edges, " %s", sha1_to_hex(b->sha1));
+						fprintf(pack_edges, " %s",
+							oid_to_hex(&b->oid));
 				}
 			}
 			for (t = first_tag; t; t = t->next_tag) {
 				if (t->pack_id == pack_id)
-					fprintf(pack_edges, " %s", sha1_to_hex(t->sha1));
+					fprintf(pack_edges, " %s",
+						oid_to_hex(&t->oid));
 			}
 			fputc('\n', pack_edges);
 			fflush(pack_edges);
@@ -1079,13 +1083,13 @@ static int store_object(
 	enum object_type type,
 	struct strbuf *dat,
 	struct last_object *last,
-	unsigned char *sha1out,
+	struct object_id *oidout,
 	uintmax_t mark)
 {
 	void *out, *delta;
 	struct object_entry *e;
 	unsigned char hdr[96];
-	unsigned char sha1[20];
+	struct object_id oid;
 	unsigned long hdrlen, deltalen;
 	git_SHA_CTX c;
 	git_zstream s;
@@ -1095,17 +1099,17 @@ static int store_object(
 	git_SHA1_Init(&c);
 	git_SHA1_Update(&c, hdr, hdrlen);
 	git_SHA1_Update(&c, dat->buf, dat->len);
-	git_SHA1_Final(sha1, &c);
-	if (sha1out)
-		hashcpy(sha1out, sha1);
+	git_SHA1_Final(oid.hash, &c);
+	if (oidout)
+		oidcpy(oidout, &oid);
 
-	e = insert_object(sha1);
+	e = insert_object(&oid);
 	if (mark)
 		insert_mark(mark, e);
 	if (e->idx.offset) {
 		duplicate_count_by_type[type]++;
 		return 1;
-	} else if (find_sha1_pack(sha1, packed_git)) {
+	} else if (find_sha1_pack(oid.hash, packed_git)) {
 		e->type = type;
 		e->pack_id = MAX_PACK_ID;
 		e->idx.offset = 1; /* just not zero! */
@@ -1218,13 +1222,13 @@ static void truncate_pack(struct sha1file_checkpoint *checkpoint)
 	pack_size = checkpoint->offset;
 }
 
-static void stream_blob(uintmax_t len, unsigned char *sha1out, uintmax_t mark)
+static void stream_blob(uintmax_t len, struct object_id *oidout, uintmax_t mark)
 {
 	size_t in_sz = 64 * 1024, out_sz = 64 * 1024;
 	unsigned char *in_buf = xmalloc(in_sz);
 	unsigned char *out_buf = xmalloc(out_sz);
 	struct object_entry *e;
-	unsigned char sha1[20];
+	struct object_id oid;
 	unsigned long hdrlen;
 	off_t offset;
 	git_SHA_CTX c;
@@ -1287,12 +1291,12 @@ static void stream_blob(uintmax_t len, unsigned char *sha1out, uintmax_t mark)
 		}
 	}
 	git_deflate_end(&s);
-	git_SHA1_Final(sha1, &c);
+	git_SHA1_Final(oid.hash, &c);
 
-	if (sha1out)
-		hashcpy(sha1out, sha1);
+	if (oidout)
+		oidcpy(oidout, &oid);
 
-	e = insert_object(sha1);
+	e = insert_object(&oid);
 
 	if (mark)
 		insert_mark(mark, e);
@@ -1301,7 +1305,7 @@ static void stream_blob(uintmax_t len, unsigned char *sha1out, uintmax_t mark)
 		duplicate_count_by_type[OBJ_BLOB]++;
 		truncate_pack(&checkpoint);
 
-	} else if (find_sha1_pack(sha1, packed_git)) {
+	} else if (find_sha1_pack(oid.hash, packed_git)) {
 		e->type = OBJ_BLOB;
 		e->pack_id = MAX_PACK_ID;
 		e->idx.offset = 1; /* just not zero! */
@@ -1385,7 +1389,7 @@ static const char *get_mode(const char *str, uint16_t *modep)
 
 static void load_tree(struct tree_entry *root)
 {
-	unsigned char *sha1 = root->versions[1].sha1;
+	struct object_id *oid = &root->versions[1].oid;
 	struct object_entry *myoe;
 	struct tree_content *t;
 	unsigned long size;
@@ -1393,22 +1397,22 @@ static void load_tree(struct tree_entry *root)
 	const char *c;
 
 	root->tree = t = new_tree_content(8);
-	if (is_null_sha1(sha1))
+	if (is_null_oid(oid))
 		return;
 
-	myoe = find_object(sha1);
+	myoe = find_object(oid);
 	if (myoe && myoe->pack_id != MAX_PACK_ID) {
 		if (myoe->type != OBJ_TREE)
-			die("Not a tree: %s", sha1_to_hex(sha1));
+			die("Not a tree: %s", oid_to_hex(oid));
 		t->delta_depth = myoe->depth;
 		buf = gfi_unpack_entry(myoe, &size);
 		if (!buf)
-			die("Can't load tree %s", sha1_to_hex(sha1));
+			die("Can't load tree %s", oid_to_hex(oid));
 	} else {
 		enum object_type type;
-		buf = read_sha1_file(sha1, &type, &size);
+		buf = read_sha1_file(oid->hash, &type, &size);
 		if (!buf || type != OBJ_TREE)
-			die("Can't load tree %s", sha1_to_hex(sha1));
+			die("Can't load tree %s", oid_to_hex(oid));
 	}
 
 	c = buf;
@@ -1422,13 +1426,13 @@ static void load_tree(struct tree_entry *root)
 		e->tree = NULL;
 		c = get_mode(c, &e->versions[1].mode);
 		if (!c)
-			die("Corrupt mode in %s", sha1_to_hex(sha1));
+			die("Corrupt mode in %s", oid_to_hex(oid));
 		e->versions[0].mode = e->versions[1].mode;
 		e->name = to_atom(c, strlen(c));
 		c += e->name->str_len + 1;
-		hashcpy(e->versions[0].sha1, (unsigned char *)c);
-		hashcpy(e->versions[1].sha1, (unsigned char *)c);
-		c += 20;
+		hashcpy(e->versions[0].oid.hash, (unsigned char *)c);
+		hashcpy(e->versions[1].oid.hash, (unsigned char *)c);
+		c += GIT_SHA1_RAWSZ;
 	}
 	free(buf);
 }
@@ -1475,7 +1479,7 @@ static void mktree(struct tree_content *t, int v, struct strbuf *b)
 		strbuf_addf(b, "%o %s%c",
 			(unsigned int)(e->versions[v].mode & ~NO_DELTA),
 			e->name->str_dat, '\0');
-		strbuf_add(b, e->versions[v].sha1, 20);
+		strbuf_add(b, e->versions[v].oid.hash, GIT_SHA1_RAWSZ);
 	}
 }
 
@@ -1486,7 +1490,7 @@ static void store_tree(struct tree_entry *root)
 	struct last_object lo = { STRBUF_INIT, 0, 0, /* no_swap */ 1 };
 	struct object_entry *le = NULL;
 
-	if (!is_null_sha1(root->versions[1].sha1))
+	if (!is_null_oid(&root->versions[1].oid))
 		return;
 
 	if (!root->tree)
@@ -1499,7 +1503,7 @@ static void store_tree(struct tree_entry *root)
 	}
 
 	if (!(root->versions[0].mode & NO_DELTA))
-		le = find_object(root->versions[0].sha1);
+		le = find_object(&root->versions[0].oid);
 	if (S_ISDIR(root->versions[0].mode) && le && le->pack_id == pack_id) {
 		mktree(t, 0, &old_tree);
 		lo.data = old_tree;
@@ -1508,14 +1512,14 @@ static void store_tree(struct tree_entry *root)
 	}
 
 	mktree(t, 1, &new_tree);
-	store_object(OBJ_TREE, &new_tree, &lo, root->versions[1].sha1, 0);
+	store_object(OBJ_TREE, &new_tree, &lo, &root->versions[1].oid, 0);
 
 	t->delta_depth = lo.depth;
 	for (i = 0, j = 0, del = 0; i < t->entry_count; i++) {
 		struct tree_entry *e = t->entries[i];
 		if (e->versions[1].mode) {
 			e->versions[0].mode = e->versions[1].mode;
-			hashcpy(e->versions[0].sha1, e->versions[1].sha1);
+			oidcpy(&e->versions[0].oid, &e->versions[1].oid);
 			t->entries[j++] = e;
 		} else {
 			release_tree_entry(e);
@@ -1527,14 +1531,14 @@ static void store_tree(struct tree_entry *root)
 
 static void tree_content_replace(
 	struct tree_entry *root,
-	const unsigned char *sha1,
+	const struct object_id *oid,
 	const uint16_t mode,
 	struct tree_content *newtree)
 {
 	if (!S_ISDIR(mode))
 		die("Root cannot be a non-directory");
-	hashclr(root->versions[0].sha1);
-	hashcpy(root->versions[1].sha1, sha1);
+	oidclr(&root->versions[0].oid);
+	oidcpy(&root->versions[1].oid, oid);
 	if (root->tree)
 		release_tree_content_recursive(root->tree);
 	root->tree = newtree;
@@ -1543,7 +1547,7 @@ static void tree_content_replace(
 static int tree_content_set(
 	struct tree_entry *root,
 	const char *p,
-	const unsigned char *sha1,
+	const struct object_id *oid,
 	const uint16_t mode,
 	struct tree_content *subtree)
 {
@@ -1568,10 +1572,10 @@ static int tree_content_set(
 			if (!*slash1) {
 				if (!S_ISDIR(mode)
 						&& e->versions[1].mode == mode
-						&& !hashcmp(e->versions[1].sha1, sha1))
+						&& !oidcmp(&e->versions[1].oid, oid))
 					return 0;
 				e->versions[1].mode = mode;
-				hashcpy(e->versions[1].sha1, sha1);
+				oidcpy(&e->versions[1].oid, oid);
 				if (e->tree)
 					release_tree_content_recursive(e->tree);
 				e->tree = subtree;
@@ -1592,7 +1596,7 @@ static int tree_content_set(
 				if (S_ISDIR(e->versions[0].mode))
 					e->versions[0].mode |= NO_DELTA;
 
-				hashclr(root->versions[1].sha1);
+				oidclr(&root->versions[1].oid);
 				return 1;
 			}
 			if (!S_ISDIR(e->versions[1].mode)) {
@@ -1601,8 +1605,8 @@ static int tree_content_set(
 			}
 			if (!e->tree)
 				load_tree(e);
-			if (tree_content_set(e, slash1 + 1, sha1, mode, subtree)) {
-				hashclr(root->versions[1].sha1);
+			if (tree_content_set(e, slash1 + 1, oid, mode, subtree)) {
+				oidclr(&root->versions[1].oid);
 				return 1;
 			}
 			return 0;
@@ -1614,18 +1618,18 @@ static int tree_content_set(
 	e = new_tree_entry();
 	e->name = to_atom(p, n);
 	e->versions[0].mode = 0;
-	hashclr(e->versions[0].sha1);
+	oidclr(&e->versions[0].oid);
 	t->entries[t->entry_count++] = e;
 	if (*slash1) {
 		e->tree = new_tree_content(8);
 		e->versions[1].mode = S_IFDIR;
-		tree_content_set(e, slash1 + 1, sha1, mode, subtree);
+		tree_content_set(e, slash1 + 1, oid, mode, subtree);
 	} else {
 		e->tree = subtree;
 		e->versions[1].mode = mode;
-		hashcpy(e->versions[1].sha1, sha1);
+		oidcpy(&e->versions[1].oid, oid);
 	}
-	hashclr(root->versions[1].sha1);
+	oidclr(&root->versions[1].oid);
 	return 1;
 }
 
@@ -1670,7 +1674,7 @@ static int tree_content_remove(
 			if (tree_content_remove(e, slash1 + 1, backup_leaf, 0)) {
 				for (n = 0; n < e->tree->entry_count; n++) {
 					if (e->tree->entries[n]->versions[1].mode) {
-						hashclr(root->versions[1].sha1);
+						oidclr(&root->versions[1].oid);
 						return 1;
 					}
 				}
@@ -1689,8 +1693,8 @@ del_entry:
 		release_tree_content_recursive(e->tree);
 	e->tree = NULL;
 	e->versions[1].mode = 0;
-	hashclr(e->versions[1].sha1);
-	hashclr(root->versions[1].sha1);
+	oidclr(&e->versions[1].oid);
+	oidclr(&root->versions[1].oid);
 	return 1;
 }
 
@@ -1735,7 +1739,7 @@ static int tree_content_get(
 
 found_entry:
 	memcpy(leaf, e, sizeof(*leaf));
-	if (e->tree && is_null_sha1(e->versions[1].sha1))
+	if (e->tree && is_null_oid(&e->versions[1].oid))
 		leaf->tree = dup_tree_content(e->tree);
 	else
 		leaf->tree = NULL;
@@ -1746,34 +1750,35 @@ static int update_branch(struct branch *b)
 {
 	static const char *msg = "fast-import";
 	struct ref_transaction *transaction;
-	unsigned char old_sha1[20];
+	struct object_id old_oid;
 	struct strbuf err = STRBUF_INIT;
 
-	if (is_null_sha1(b->sha1)) {
+	if (is_null_oid(&b->oid)) {
 		if (b->delete)
 			delete_ref(NULL, b->name, NULL, 0);
 		return 0;
 	}
-	if (read_ref(b->name, old_sha1))
-		hashclr(old_sha1);
-	if (!force_update && !is_null_sha1(old_sha1)) {
+	if (read_ref(b->name, old_oid.hash))
+		oidclr(&old_oid);
+	if (!force_update && !is_null_oid(&old_oid)) {
 		struct commit *old_cmit, *new_cmit;
 
-		old_cmit = lookup_commit_reference_gently(old_sha1, 0);
-		new_cmit = lookup_commit_reference_gently(b->sha1, 0);
+		old_cmit = lookup_commit_reference_gently(&old_oid, 0);
+		new_cmit = lookup_commit_reference_gently(&b->oid, 0);
 		if (!old_cmit || !new_cmit)
 			return error("Branch %s is missing commits.", b->name);
 
 		if (!in_merge_bases(old_cmit, new_cmit)) {
 			warning("Not updating %s"
 				" (new tip %s does not contain %s)",
-				b->name, sha1_to_hex(b->sha1), sha1_to_hex(old_sha1));
+				b->name, oid_to_hex(&b->oid),
+				oid_to_hex(&old_oid));
 			return -1;
 		}
 	}
 	transaction = ref_transaction_begin(&err);
 	if (!transaction ||
-	    ref_transaction_update(transaction, b->name, b->sha1, old_sha1,
+	    ref_transaction_update(transaction, b->name, b->oid.hash, old_oid.hash,
 				   0, msg, &err) ||
 	    ref_transaction_commit(transaction, &err)) {
 		ref_transaction_free(transaction);
@@ -1815,7 +1820,7 @@ static void dump_tags(void)
 		strbuf_addf(&ref_name, "refs/tags/%s", t->name);
 
 		if (ref_transaction_update(transaction, ref_name.buf,
-					   t->sha1, NULL, 0, msg, &err)) {
+					   t->oid.hash, NULL, 0, msg, &err)) {
 			failure |= error("%s", err.buf);
 			goto cleanup;
 		}
@@ -1844,7 +1849,7 @@ static void dump_marks_helper(FILE *f,
 		for (k = 0; k < 1024; k++) {
 			if (m->data.marked[k])
 				fprintf(f, ":%" PRIuMAX " %s\n", base + k,
-					sha1_to_hex(m->data.marked[k]->idx.sha1));
+					oid_to_hex(&m->data.marked[k]->idx.oid));
 		}
 	}
 }
@@ -1893,7 +1898,7 @@ static void read_marks(void)
 	while (fgets(line, sizeof(line), f)) {
 		uintmax_t mark;
 		char *end;
-		unsigned char sha1[20];
+		struct object_id oid;
 		struct object_entry *e;
 
 		end = strchr(line, '\n');
@@ -1902,14 +1907,14 @@ static void read_marks(void)
 		*end = 0;
 		mark = strtoumax(line + 1, &end, 10);
 		if (!mark || end == line + 1
-			|| *end != ' ' || get_sha1_hex(end + 1, sha1))
+			|| *end != ' ' || get_oid_hex(end + 1, &oid))
 			die("corrupt mark line: %s", line);
-		e = find_object(sha1);
+		e = find_object(&oid);
 		if (!e) {
-			enum object_type type = sha1_object_info(sha1, NULL);
+			enum object_type type = sha1_object_info(oid.hash, NULL);
 			if (type < 0)
-				die("object not found: %s", sha1_to_hex(sha1));
-			e = insert_object(sha1);
+				die("object not found: %s", oid_to_hex(&oid));
+			e = insert_object(&oid);
 			e->type = type;
 			e->pack_id = MAX_PACK_ID;
 			e->idx.offset = 1; /* just not zero! */
@@ -2117,21 +2122,21 @@ static char *parse_ident(const char *buf)
 
 static void parse_and_store_blob(
 	struct last_object *last,
-	unsigned char *sha1out,
+	struct object_id *oidout,
 	uintmax_t mark)
 {
 	static struct strbuf buf = STRBUF_INIT;
 	uintmax_t len;
 
 	if (parse_data(&buf, big_file_threshold, &len))
-		store_object(OBJ_BLOB, &buf, last, sha1out, mark);
+		store_object(OBJ_BLOB, &buf, last, oidout, mark);
 	else {
 		if (last) {
 			strbuf_release(&last->data);
 			last->offset = 0;
 			last->depth = 0;
 		}
-		stream_blob(len, sha1out, mark);
+		stream_blob(len, oidout, mark);
 		skip_optional_lf();
 	}
 }
@@ -2207,21 +2212,21 @@ static void construct_path_with_fanout(const char *hex_sha1,
 		path[i++] = '/';
 		fanout--;
 	}
-	memcpy(path + i, hex_sha1 + j, 40 - j);
-	path[i + 40 - j] = '\0';
+	memcpy(path + i, hex_sha1 + j, GIT_SHA1_HEXSZ - j);
+	path[i + GIT_SHA1_HEXSZ - j] = '\0';
 }
 
 static uintmax_t do_change_note_fanout(
 		struct tree_entry *orig_root, struct tree_entry *root,
-		char *hex_sha1, unsigned int hex_sha1_len,
+		char *hex_oid, unsigned int hex_oid_len,
 		char *fullpath, unsigned int fullpath_len,
 		unsigned char fanout)
 {
 	struct tree_content *t;
 	struct tree_entry *e, leaf;
-	unsigned int i, tmp_hex_sha1_len, tmp_fullpath_len;
+	unsigned int i, tmp_hex_oid_len, tmp_fullpath_len;
 	uintmax_t num_notes = 0;
-	unsigned char sha1[20];
+	struct object_id oid;
 	char realpath[60];
 
 	if (!root->tree)
@@ -2230,7 +2235,7 @@ static uintmax_t do_change_note_fanout(
 
 	for (i = 0; t && i < t->entry_count; i++) {
 		e = t->entries[i];
-		tmp_hex_sha1_len = hex_sha1_len + e->name->str_len;
+		tmp_hex_oid_len = hex_oid_len + e->name->str_len;
 		tmp_fullpath_len = fullpath_len;
 
 		/*
@@ -2242,12 +2247,12 @@ static uintmax_t do_change_note_fanout(
 		 * of 2 chars.
 		 */
 		if (!e->versions[1].mode ||
-		    tmp_hex_sha1_len > 40 ||
+		    tmp_hex_oid_len > GIT_SHA1_HEXSZ ||
 		    e->name->str_len % 2)
 			continue;
 
 		/* This _may_ be a note entry, or a subdir containing notes */
-		memcpy(hex_sha1 + hex_sha1_len, e->name->str_dat,
+		memcpy(hex_oid + hex_oid_len, e->name->str_dat,
 		       e->name->str_len);
 		if (tmp_fullpath_len)
 			fullpath[tmp_fullpath_len++] = '/';
@@ -2256,14 +2261,14 @@ static uintmax_t do_change_note_fanout(
 		tmp_fullpath_len += e->name->str_len;
 		fullpath[tmp_fullpath_len] = '\0';
 
-		if (tmp_hex_sha1_len == 40 && !get_sha1_hex(hex_sha1, sha1)) {
+		if (tmp_hex_oid_len == GIT_SHA1_HEXSZ && !get_oid_hex(hex_oid, &oid)) {
 			/* This is a note entry */
 			if (fanout == 0xff) {
 				/* Counting mode, no rename */
 				num_notes++;
 				continue;
 			}
-			construct_path_with_fanout(hex_sha1, fanout, realpath);
+			construct_path_with_fanout(hex_oid, fanout, realpath);
 			if (!strcmp(fullpath, realpath)) {
 				/* Note entry is in correct location */
 				num_notes++;
@@ -2274,13 +2279,13 @@ static uintmax_t do_change_note_fanout(
 			if (!tree_content_remove(orig_root, fullpath, &leaf, 0))
 				die("Failed to remove path %s", fullpath);
 			tree_content_set(orig_root, realpath,
-				leaf.versions[1].sha1,
+				&leaf.versions[1].oid,
 				leaf.versions[1].mode,
 				leaf.tree);
 		} else if (S_ISDIR(e->versions[1].mode)) {
 			/* This is a subdir that may contain note entries */
 			num_notes += do_change_note_fanout(orig_root, e,
-				hex_sha1, tmp_hex_sha1_len,
+				hex_oid, tmp_hex_oid_len,
 				fullpath, tmp_fullpath_len, fanout);
 		}
 
@@ -2293,8 +2298,14 @@ static uintmax_t do_change_note_fanout(
 static uintmax_t change_note_fanout(struct tree_entry *root,
 		unsigned char fanout)
 {
-	char hex_sha1[40], path[60];
-	return do_change_note_fanout(root, root, hex_sha1, 0, path, 0, fanout);
+	/*
+	 * The size of path is due to one slash between every two hex digits,
+	 * plus the terminating NUL.  Note that there is no slash at the end, so
+	 * the number of slashes is one less than half the number of hex
+	 * characters.
+	 */
+	char hex_oid[GIT_MAX_HEXSZ], path[GIT_MAX_HEXSZ + (GIT_MAX_HEXSZ / 2) - 1 + 1];
+	return do_change_note_fanout(root, root, hex_oid, 0, path, 0, fanout);
 }
 
 /*
@@ -2355,7 +2366,7 @@ static void file_change_m(const char *p, struct branch *b)
 	static struct strbuf uq = STRBUF_INIT;
 	const char *endp;
 	struct object_entry *oe;
-	unsigned char sha1[20];
+	struct object_id oid;
 	uint16_t mode, inline_data = 0;
 
 	p = get_mode(p, &mode);
@@ -2378,15 +2389,14 @@ static void file_change_m(const char *p, struct branch *b)
 
 	if (*p == ':') {
 		oe = find_mark(parse_mark_ref_space(&p));
-		hashcpy(sha1, oe->idx.sha1);
+		oidcpy(&oid, &oe->idx.oid);
 	} else if (skip_prefix(p, "inline ", &p)) {
 		inline_data = 1;
 		oe = NULL; /* not used with inline_data, but makes gcc happy */
 	} else {
-		if (get_sha1_hex(p, sha1))
+		if (parse_oid_hex(p, &oid, &p))
 			die("Invalid dataref: %s", command_buf.buf);
-		oe = find_object(sha1);
-		p += 40;
+		oe = find_object(&oid);
 		if (*p++ != ' ')
 			die("Missing space after SHA1: %s", command_buf.buf);
 	}
@@ -2399,7 +2409,7 @@ static void file_change_m(const char *p, struct branch *b)
 	}
 
 	/* Git does not track empty, non-toplevel directories. */
-	if (S_ISDIR(mode) && !hashcmp(sha1, EMPTY_TREE_SHA1_BIN) && *p) {
+	if (S_ISDIR(mode) && is_empty_tree_oid(&oid) && *p) {
 		tree_content_remove(&b->branch_tree, p, NULL, 0);
 		return;
 	}
@@ -2426,12 +2436,12 @@ static void file_change_m(const char *p, struct branch *b)
 			p = uq.buf;
 		}
 		read_next_command();
-		parse_and_store_blob(&last_blob, sha1, 0);
+		parse_and_store_blob(&last_blob, &oid, 0);
 	} else {
 		enum object_type expected = S_ISDIR(mode) ?
 						OBJ_TREE: OBJ_BLOB;
 		enum object_type type = oe ? oe->type :
-					sha1_object_info(sha1, NULL);
+					sha1_object_info(oid.hash, NULL);
 		if (type < 0)
 			die("%s not found: %s",
 					S_ISDIR(mode) ?  "Tree" : "Blob",
@@ -2443,10 +2453,10 @@ static void file_change_m(const char *p, struct branch *b)
 	}
 
 	if (!*p) {
-		tree_content_replace(&b->branch_tree, sha1, mode, NULL);
+		tree_content_replace(&b->branch_tree, &oid, mode, NULL);
 		return;
 	}
-	tree_content_set(&b->branch_tree, p, sha1, mode, NULL);
+	tree_content_set(&b->branch_tree, p, &oid, mode, NULL);
 }
 
 static void file_change_d(const char *p, struct branch *b)
@@ -2504,13 +2514,13 @@ static void file_change_cr(const char *s, struct branch *b, int rename)
 		die("Path %s not in branch", s);
 	if (!*d) {	/* C "path/to/subdir" "" */
 		tree_content_replace(&b->branch_tree,
-			leaf.versions[1].sha1,
+			&leaf.versions[1].oid,
 			leaf.versions[1].mode,
 			leaf.tree);
 		return;
 	}
 	tree_content_set(&b->branch_tree, d,
-		leaf.versions[1].sha1,
+		&leaf.versions[1].oid,
 		leaf.versions[1].mode,
 		leaf.tree);
 }
@@ -2520,7 +2530,7 @@ static void note_change_n(const char *p, struct branch *b, unsigned char *old_fa
 	static struct strbuf uq = STRBUF_INIT;
 	struct object_entry *oe;
 	struct branch *s;
-	unsigned char sha1[20], commit_sha1[20];
+	struct object_id oid, commit_oid;
 	char path[60];
 	uint16_t inline_data = 0;
 	unsigned char new_fanout;
@@ -2545,15 +2555,14 @@ static void note_change_n(const char *p, struct branch *b, unsigned char *old_fa
 	/* <dataref> or 'inline' */
 	if (*p == ':') {
 		oe = find_mark(parse_mark_ref_space(&p));
-		hashcpy(sha1, oe->idx.sha1);
+		oidcpy(&oid, &oe->idx.oid);
 	} else if (skip_prefix(p, "inline ", &p)) {
 		inline_data = 1;
 		oe = NULL; /* not used with inline_data, but makes gcc happy */
 	} else {
-		if (get_sha1_hex(p, sha1))
+		if (parse_oid_hex(p, &oid, &p))
 			die("Invalid dataref: %s", command_buf.buf);
-		oe = find_object(sha1);
-		p += 40;
+		oe = find_object(&oid);
 		if (*p++ != ' ')
 			die("Missing space after SHA1: %s", command_buf.buf);
 	}
@@ -2561,19 +2570,19 @@ static void note_change_n(const char *p, struct branch *b, unsigned char *old_fa
 	/* <commit-ish> */
 	s = lookup_branch(p);
 	if (s) {
-		if (is_null_sha1(s->sha1))
+		if (is_null_oid(&s->oid))
 			die("Can't add a note on empty branch.");
-		hashcpy(commit_sha1, s->sha1);
+		oidcpy(&commit_oid, &s->oid);
 	} else if (*p == ':') {
 		uintmax_t commit_mark = parse_mark_ref_eol(p);
 		struct object_entry *commit_oe = find_mark(commit_mark);
 		if (commit_oe->type != OBJ_COMMIT)
 			die("Mark :%" PRIuMAX " not a commit", commit_mark);
-		hashcpy(commit_sha1, commit_oe->idx.sha1);
-	} else if (!get_sha1(p, commit_sha1)) {
+		oidcpy(&commit_oid, &commit_oe->idx.oid);
+	} else if (!get_oid(p, &commit_oid)) {
 		unsigned long size;
-		char *buf = read_object_with_reference(commit_sha1,
-			commit_type, &size, commit_sha1);
+		char *buf = read_object_with_reference(commit_oid.hash,
+			commit_type, &size, commit_oid.hash);
 		if (!buf || size < 46)
 			die("Not a valid commit: %s", p);
 		free(buf);
@@ -2586,13 +2595,13 @@ static void note_change_n(const char *p, struct branch *b, unsigned char *old_fa
 			p = uq.buf;
 		}
 		read_next_command();
-		parse_and_store_blob(&last_blob, sha1, 0);
+		parse_and_store_blob(&last_blob, &oid, 0);
 	} else if (oe) {
 		if (oe->type != OBJ_BLOB)
 			die("Not a blob (actually a %s): %s",
 				typename(oe->type), command_buf.buf);
-	} else if (!is_null_sha1(sha1)) {
-		enum object_type type = sha1_object_info(sha1, NULL);
+	} else if (!is_null_oid(&oid)) {
+		enum object_type type = sha1_object_info(oid.hash, NULL);
 		if (type < 0)
 			die("Blob not found: %s", command_buf.buf);
 		if (type != OBJ_BLOB)
@@ -2600,50 +2609,51 @@ static void note_change_n(const char *p, struct branch *b, unsigned char *old_fa
 			    typename(type), command_buf.buf);
 	}
 
-	construct_path_with_fanout(sha1_to_hex(commit_sha1), *old_fanout, path);
+	construct_path_with_fanout(oid_to_hex(&commit_oid), *old_fanout, path);
 	if (tree_content_remove(&b->branch_tree, path, NULL, 0))
 		b->num_notes--;
 
-	if (is_null_sha1(sha1))
+	if (is_null_oid(&oid))
 		return; /* nothing to insert */
 
 	b->num_notes++;
 	new_fanout = convert_num_notes_to_fanout(b->num_notes);
-	construct_path_with_fanout(sha1_to_hex(commit_sha1), new_fanout, path);
-	tree_content_set(&b->branch_tree, path, sha1, S_IFREG | 0644, NULL);
+	construct_path_with_fanout(oid_to_hex(&commit_oid), new_fanout, path);
+	tree_content_set(&b->branch_tree, path, &oid, S_IFREG | 0644, NULL);
 }
 
 static void file_change_deleteall(struct branch *b)
 {
 	release_tree_content_recursive(b->branch_tree.tree);
-	hashclr(b->branch_tree.versions[0].sha1);
-	hashclr(b->branch_tree.versions[1].sha1);
+	oidclr(&b->branch_tree.versions[0].oid);
+	oidclr(&b->branch_tree.versions[1].oid);
 	load_tree(&b->branch_tree);
 	b->num_notes = 0;
 }
 
 static void parse_from_commit(struct branch *b, char *buf, unsigned long size)
 {
-	if (!buf || size < 46)
-		die("Not a valid commit: %s", sha1_to_hex(b->sha1));
+	if (!buf || size < GIT_SHA1_HEXSZ + 6)
+		die("Not a valid commit: %s", oid_to_hex(&b->oid));
 	if (memcmp("tree ", buf, 5)
-		|| get_sha1_hex(buf + 5, b->branch_tree.versions[1].sha1))
-		die("The commit %s is corrupt", sha1_to_hex(b->sha1));
-	hashcpy(b->branch_tree.versions[0].sha1,
-		b->branch_tree.versions[1].sha1);
+		|| get_oid_hex(buf + 5, &b->branch_tree.versions[1].oid))
+		die("The commit %s is corrupt", oid_to_hex(&b->oid));
+	oidcpy(&b->branch_tree.versions[0].oid,
+	       &b->branch_tree.versions[1].oid);
 }
 
 static void parse_from_existing(struct branch *b)
 {
-	if (is_null_sha1(b->sha1)) {
-		hashclr(b->branch_tree.versions[0].sha1);
-		hashclr(b->branch_tree.versions[1].sha1);
+	if (is_null_oid(&b->oid)) {
+		oidclr(&b->branch_tree.versions[0].oid);
+		oidclr(&b->branch_tree.versions[1].oid);
 	} else {
 		unsigned long size;
 		char *buf;
 
-		buf = read_object_with_reference(b->sha1,
-			commit_type, &size, b->sha1);
+		buf = read_object_with_reference(b->oid.hash,
+						 commit_type, &size,
+						 b->oid.hash);
 		parse_from_commit(b, buf, size);
 		free(buf);
 	}
@@ -2653,28 +2663,28 @@ static int parse_from(struct branch *b)
 {
 	const char *from;
 	struct branch *s;
-	unsigned char sha1[20];
+	struct object_id oid;
 
 	if (!skip_prefix(command_buf.buf, "from ", &from))
 		return 0;
 
-	hashcpy(sha1, b->branch_tree.versions[1].sha1);
+	oidcpy(&oid, &b->branch_tree.versions[1].oid);
 
 	s = lookup_branch(from);
 	if (b == s)
 		die("Can't create a branch from itself: %s", b->name);
 	else if (s) {
-		unsigned char *t = s->branch_tree.versions[1].sha1;
-		hashcpy(b->sha1, s->sha1);
-		hashcpy(b->branch_tree.versions[0].sha1, t);
-		hashcpy(b->branch_tree.versions[1].sha1, t);
+		struct object_id *t = &s->branch_tree.versions[1].oid;
+		oidcpy(&b->oid, &s->oid);
+		oidcpy(&b->branch_tree.versions[0].oid, t);
+		oidcpy(&b->branch_tree.versions[1].oid, t);
 	} else if (*from == ':') {
 		uintmax_t idnum = parse_mark_ref_eol(from);
 		struct object_entry *oe = find_mark(idnum);
 		if (oe->type != OBJ_COMMIT)
 			die("Mark :%" PRIuMAX " not a commit", idnum);
-		if (hashcmp(b->sha1, oe->idx.sha1)) {
-			hashcpy(b->sha1, oe->idx.sha1);
+		if (oidcmp(&b->oid, &oe->idx.oid)) {
+			oidcpy(&b->oid, &oe->idx.oid);
 			if (oe->pack_id != MAX_PACK_ID) {
 				unsigned long size;
 				char *buf = gfi_unpack_entry(oe, &size);
@@ -2683,15 +2693,15 @@ static int parse_from(struct branch *b)
 			} else
 				parse_from_existing(b);
 		}
-	} else if (!get_sha1(from, b->sha1)) {
+	} else if (!get_oid(from, &b->oid)) {
 		parse_from_existing(b);
-		if (is_null_sha1(b->sha1))
+		if (is_null_oid(&b->oid))
 			b->delete = 1;
 	}
 	else
 		die("Invalid ref name or SHA1 expression: %s", from);
 
-	if (b->branch_tree.tree && hashcmp(sha1, b->branch_tree.versions[1].sha1)) {
+	if (b->branch_tree.tree && oidcmp(&oid, &b->branch_tree.versions[1].oid)) {
 		release_tree_content_recursive(b->branch_tree.tree);
 		b->branch_tree.tree = NULL;
 	}
@@ -2711,17 +2721,17 @@ static struct hash_list *parse_merge(unsigned int *count)
 		n = xmalloc(sizeof(*n));
 		s = lookup_branch(from);
 		if (s)
-			hashcpy(n->sha1, s->sha1);
+			oidcpy(&n->oid, &s->oid);
 		else if (*from == ':') {
 			uintmax_t idnum = parse_mark_ref_eol(from);
 			struct object_entry *oe = find_mark(idnum);
 			if (oe->type != OBJ_COMMIT)
 				die("Mark :%" PRIuMAX " not a commit", idnum);
-			hashcpy(n->sha1, oe->idx.sha1);
-		} else if (!get_sha1(from, n->sha1)) {
+			oidcpy(&n->oid, &oe->idx.oid);
+		} else if (!get_oid(from, &n->oid)) {
 			unsigned long size;
-			char *buf = read_object_with_reference(n->sha1,
-				commit_type, &size, n->sha1);
+			char *buf = read_object_with_reference(n->oid.hash,
+				commit_type, &size, n->oid.hash);
 			if (!buf || size < 46)
 				die("Not a valid commit: %s", from);
 			free(buf);
@@ -2808,17 +2818,19 @@ static void parse_new_commit(const char *arg)
 
 	/* build the tree and the commit */
 	store_tree(&b->branch_tree);
-	hashcpy(b->branch_tree.versions[0].sha1,
-		b->branch_tree.versions[1].sha1);
+	oidcpy(&b->branch_tree.versions[0].oid,
+	       &b->branch_tree.versions[1].oid);
 
 	strbuf_reset(&new_data);
 	strbuf_addf(&new_data, "tree %s\n",
-		sha1_to_hex(b->branch_tree.versions[1].sha1));
-	if (!is_null_sha1(b->sha1))
-		strbuf_addf(&new_data, "parent %s\n", sha1_to_hex(b->sha1));
+		oid_to_hex(&b->branch_tree.versions[1].oid));
+	if (!is_null_oid(&b->oid))
+		strbuf_addf(&new_data, "parent %s\n",
+			    oid_to_hex(&b->oid));
 	while (merge_list) {
 		struct hash_list *next = merge_list->next;
-		strbuf_addf(&new_data, "parent %s\n", sha1_to_hex(merge_list->sha1));
+		strbuf_addf(&new_data, "parent %s\n",
+			    oid_to_hex(&merge_list->oid));
 		free(merge_list);
 		merge_list = next;
 	}
@@ -2831,7 +2843,7 @@ static void parse_new_commit(const char *arg)
 	free(author);
 	free(committer);
 
-	if (!store_object(OBJ_COMMIT, &new_data, NULL, b->sha1, next_mark))
+	if (!store_object(OBJ_COMMIT, &new_data, NULL, &b->oid, next_mark))
 		b->pack_id = pack_id;
 	b->last_commit = object_count_by_type[OBJ_COMMIT];
 }
@@ -2844,7 +2856,7 @@ static void parse_new_tag(const char *arg)
 	struct branch *s;
 	struct tag *t;
 	uintmax_t from_mark = 0;
-	unsigned char sha1[20];
+	struct object_id oid;
 	enum object_type type;
 	const char *v;
 
@@ -2863,20 +2875,20 @@ static void parse_new_tag(const char *arg)
 		die("Expected from command, got %s", command_buf.buf);
 	s = lookup_branch(from);
 	if (s) {
-		if (is_null_sha1(s->sha1))
+		if (is_null_oid(&s->oid))
 			die("Can't tag an empty branch.");
-		hashcpy(sha1, s->sha1);
+		oidcpy(&oid, &s->oid);
 		type = OBJ_COMMIT;
 	} else if (*from == ':') {
 		struct object_entry *oe;
 		from_mark = parse_mark_ref_eol(from);
 		oe = find_mark(from_mark);
 		type = oe->type;
-		hashcpy(sha1, oe->idx.sha1);
-	} else if (!get_sha1(from, sha1)) {
-		struct object_entry *oe = find_object(sha1);
+		oidcpy(&oid, &oe->idx.oid);
+	} else if (!get_oid(from, &oid)) {
+		struct object_entry *oe = find_object(&oid);
 		if (!oe) {
-			type = sha1_object_info(sha1, NULL);
+			type = sha1_object_info(oid.hash, NULL);
 			if (type < 0)
 				die("Not a valid object: %s", from);
 		} else
@@ -2902,7 +2914,7 @@ static void parse_new_tag(const char *arg)
 		    "object %s\n"
 		    "type %s\n"
 		    "tag %s\n",
-		    sha1_to_hex(sha1), typename(type), t->name);
+		    oid_to_hex(&oid), typename(type), t->name);
 	if (tagger)
 		strbuf_addf(&new_data,
 			    "tagger %s\n", tagger);
@@ -2910,7 +2922,7 @@ static void parse_new_tag(const char *arg)
 	strbuf_addbuf(&new_data, &msg);
 	free(tagger);
 
-	if (store_object(OBJ_TAG, &new_data, NULL, t->sha1, 0))
+	if (store_object(OBJ_TAG, &new_data, NULL, &t->oid, 0))
 		t->pack_id = MAX_PACK_ID;
 	else
 		t->pack_id = pack_id;
@@ -2922,9 +2934,9 @@ static void parse_reset_branch(const char *arg)
 
 	b = lookup_branch(arg);
 	if (b) {
-		hashclr(b->sha1);
-		hashclr(b->branch_tree.versions[0].sha1);
-		hashclr(b->branch_tree.versions[1].sha1);
+		oidclr(&b->oid);
+		oidclr(&b->branch_tree.versions[0].oid);
+		oidclr(&b->branch_tree.versions[1].oid);
 		if (b->branch_tree.tree) {
 			release_tree_content_recursive(b->branch_tree.tree);
 			b->branch_tree.tree = NULL;
@@ -2944,7 +2956,7 @@ static void cat_blob_write(const char *buf, unsigned long size)
 		die_errno("Write to frontend failed");
 }
 
-static void cat_blob(struct object_entry *oe, unsigned char sha1[20])
+static void cat_blob(struct object_entry *oe, struct object_id *oid)
 {
 	struct strbuf line = STRBUF_INIT;
 	unsigned long size;
@@ -2952,7 +2964,7 @@ static void cat_blob(struct object_entry *oe, unsigned char sha1[20])
 	char *buf;
 
 	if (!oe || oe->pack_id == MAX_PACK_ID) {
-		buf = read_sha1_file(sha1, &type, &size);
+		buf = read_sha1_file(oid->hash, &type, &size);
 	} else {
 		type = oe->type;
 		buf = gfi_unpack_entry(oe, &size);
@@ -2963,19 +2975,19 @@ static void cat_blob(struct object_entry *oe, unsigned char sha1[20])
 	 */
 	if (type <= 0) {
 		strbuf_reset(&line);
-		strbuf_addf(&line, "%s missing\n", sha1_to_hex(sha1));
+		strbuf_addf(&line, "%s missing\n", oid_to_hex(oid));
 		cat_blob_write(line.buf, line.len);
 		strbuf_release(&line);
 		free(buf);
 		return;
 	}
 	if (!buf)
-		die("Can't read object %s", sha1_to_hex(sha1));
+		die("Can't read object %s", oid_to_hex(oid));
 	if (type != OBJ_BLOB)
 		die("Object %s is a %s but a blob was expected.",
-		    sha1_to_hex(sha1), typename(type));
+		    oid_to_hex(oid), typename(type));
 	strbuf_reset(&line);
-	strbuf_addf(&line, "%s %s %lu\n", sha1_to_hex(sha1),
+	strbuf_addf(&line, "%s %s %lu\n", oid_to_hex(oid),
 						typename(type), size);
 	cat_blob_write(line.buf, line.len);
 	strbuf_release(&line);
@@ -2992,7 +3004,7 @@ static void cat_blob(struct object_entry *oe, unsigned char sha1[20])
 static void parse_get_mark(const char *p)
 {
 	struct object_entry *oe = oe;
-	char output[42];
+	char output[GIT_MAX_HEXSZ + 2];
 
 	/* get-mark SP <object> LF */
 	if (*p != ':')
@@ -3002,43 +3014,43 @@ static void parse_get_mark(const char *p)
 	if (!oe)
 		die("Unknown mark: %s", command_buf.buf);
 
-	xsnprintf(output, sizeof(output), "%s\n", sha1_to_hex(oe->idx.sha1));
-	cat_blob_write(output, 41);
+	xsnprintf(output, sizeof(output), "%s\n", oid_to_hex(&oe->idx.oid));
+	cat_blob_write(output, GIT_SHA1_HEXSZ + 1);
 }
 
 static void parse_cat_blob(const char *p)
 {
 	struct object_entry *oe = oe;
-	unsigned char sha1[20];
+	struct object_id oid;
 
 	/* cat-blob SP <object> LF */
 	if (*p == ':') {
 		oe = find_mark(parse_mark_ref_eol(p));
 		if (!oe)
 			die("Unknown mark: %s", command_buf.buf);
-		hashcpy(sha1, oe->idx.sha1);
+		oidcpy(&oid, &oe->idx.oid);
 	} else {
-		if (get_sha1_hex(p, sha1))
+		if (parse_oid_hex(p, &oid, &p))
 			die("Invalid dataref: %s", command_buf.buf);
-		if (p[40])
+		if (*p)
 			die("Garbage after SHA1: %s", command_buf.buf);
-		oe = find_object(sha1);
+		oe = find_object(&oid);
 	}
 
-	cat_blob(oe, sha1);
+	cat_blob(oe, &oid);
 }
 
 static struct object_entry *dereference(struct object_entry *oe,
-					unsigned char sha1[20])
+					struct object_id *oid)
 {
 	unsigned long size;
 	char *buf = NULL;
 	if (!oe) {
-		enum object_type type = sha1_object_info(sha1, NULL);
+		enum object_type type = sha1_object_info(oid->hash, NULL);
 		if (type < 0)
-			die("object not found: %s", sha1_to_hex(sha1));
+			die("object not found: %s", oid_to_hex(oid));
 		/* cache it! */
-		oe = insert_object(sha1);
+		oe = insert_object(oid);
 		oe->type = type;
 		oe->pack_id = MAX_PACK_ID;
 		oe->idx.offset = 1;
@@ -3057,49 +3069,48 @@ static struct object_entry *dereference(struct object_entry *oe,
 		buf = gfi_unpack_entry(oe, &size);
 	} else {
 		enum object_type unused;
-		buf = read_sha1_file(sha1, &unused, &size);
+		buf = read_sha1_file(oid->hash, &unused, &size);
 	}
 	if (!buf)
-		die("Can't load object %s", sha1_to_hex(sha1));
+		die("Can't load object %s", oid_to_hex(oid));
 
 	/* Peel one layer. */
 	switch (oe->type) {
 	case OBJ_TAG:
-		if (size < 40 + strlen("object ") ||
-		    get_sha1_hex(buf + strlen("object "), sha1))
+		if (size < GIT_SHA1_HEXSZ + strlen("object ") ||
+		    get_oid_hex(buf + strlen("object "), oid))
 			die("Invalid SHA1 in tag: %s", command_buf.buf);
 		break;
 	case OBJ_COMMIT:
-		if (size < 40 + strlen("tree ") ||
-		    get_sha1_hex(buf + strlen("tree "), sha1))
+		if (size < GIT_SHA1_HEXSZ + strlen("tree ") ||
+		    get_oid_hex(buf + strlen("tree "), oid))
 			die("Invalid SHA1 in commit: %s", command_buf.buf);
 	}
 
 	free(buf);
-	return find_object(sha1);
+	return find_object(oid);
 }
 
 static struct object_entry *parse_treeish_dataref(const char **p)
 {
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct object_entry *e;
 
 	if (**p == ':') {	/* <mark> */
 		e = find_mark(parse_mark_ref_space(p));
 		if (!e)
 			die("Unknown mark: %s", command_buf.buf);
-		hashcpy(sha1, e->idx.sha1);
+		oidcpy(&oid, &e->idx.oid);
 	} else {	/* <sha1> */
-		if (get_sha1_hex(*p, sha1))
+		if (parse_oid_hex(*p, &oid, p))
 			die("Invalid dataref: %s", command_buf.buf);
-		e = find_object(sha1);
-		*p += 40;
+		e = find_object(&oid);
 		if (*(*p)++ != ' ')
 			die("Missing space after tree-ish: %s", command_buf.buf);
 	}
 
 	while (!e || e->type != OBJ_TREE)
-		e = dereference(e, sha1);
+		e = dereference(e, &oid);
 	return e;
 }
 
@@ -3143,8 +3154,8 @@ static void parse_ls(const char *p, struct branch *b)
 	} else {
 		struct object_entry *e = parse_treeish_dataref(&p);
 		root = new_tree_entry();
-		hashcpy(root->versions[1].sha1, e->idx.sha1);
-		if (!is_null_sha1(root->versions[1].sha1))
+		oidcpy(&root->versions[1].oid, &e->idx.oid);
+		if (!is_null_oid(&root->versions[1].oid))
 			root->versions[1].mode = S_IFDIR;
 		load_tree(root);
 	}
@@ -3166,7 +3177,7 @@ static void parse_ls(const char *p, struct branch *b)
 	if (S_ISDIR(leaf.versions[1].mode))
 		store_tree(&leaf);
 
-	print_ls(leaf.versions[1].mode, leaf.versions[1].sha1, p);
+	print_ls(leaf.versions[1].mode, leaf.versions[1].oid.hash, p);
 	if (leaf.tree)
 		release_tree_content_recursive(leaf.tree);
 	if (!b || root != &b->branch_tree)
diff --git a/fetch-pack.c b/fetch-pack.c
index 5f15dd2c39..44fa047bf4 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -78,7 +78,7 @@ static void cache_one_alternate(const char *refname,
 				void *vcache)
 {
 	struct alternate_object_cache *cache = vcache;
-	struct object *obj = parse_object(oid->hash);
+	struct object *obj = parse_object(oid);
 
 	if (!obj || (obj->flags & ALTERNATE))
 		return;
@@ -118,9 +118,9 @@ static void rev_list_push(struct commit *commit, int mark)
 	}
 }
 
-static int rev_list_insert_ref(const char *refname, const unsigned char *sha1)
+static int rev_list_insert_ref(const char *refname, const struct object_id *oid)
 {
-	struct object *o = deref_tag(parse_object(sha1), refname, 0);
+	struct object *o = deref_tag(parse_object(oid), refname, 0);
 
 	if (o && o->type == OBJ_COMMIT)
 		rev_list_push((struct commit *)o, SEEN);
@@ -131,13 +131,13 @@ static int rev_list_insert_ref(const char *refname, const unsigned char *sha1)
 static int rev_list_insert_ref_oid(const char *refname, const struct object_id *oid,
 				   int flag, void *cb_data)
 {
-	return rev_list_insert_ref(refname, oid->hash);
+	return rev_list_insert_ref(refname, oid);
 }
 
 static int clear_marks(const char *refname, const struct object_id *oid,
 		       int flag, void *cb_data)
 {
-	struct object *o = deref_tag(parse_object(oid->hash), refname, 0);
+	struct object *o = deref_tag(parse_object(oid), refname, 0);
 
 	if (o && o->type == OBJ_COMMIT)
 		clear_commit_marks((struct commit *)o,
@@ -183,7 +183,7 @@ static void mark_common(struct commit *commit,
   Get the next rev to send, ignoring the common.
 */
 
-static const unsigned char *get_rev(void)
+static const struct object_id *get_rev(void)
 {
 	struct commit *commit = NULL;
 
@@ -222,7 +222,7 @@ static const unsigned char *get_rev(void)
 		}
 	}
 
-	return commit->object.oid.hash;
+	return &commit->object.oid;
 }
 
 enum ack_type {
@@ -251,7 +251,7 @@ static void consume_shallow_list(struct fetch_pack_args *args, int fd)
 	}
 }
 
-static enum ack_type get_ack(int fd, unsigned char *result_sha1)
+static enum ack_type get_ack(int fd, struct object_id *result_oid)
 {
 	int len;
 	char *line = packet_read_line(fd, &len);
@@ -262,7 +262,7 @@ static enum ack_type get_ack(int fd, unsigned char *result_sha1)
 	if (!strcmp(line, "NAK"))
 		return NAK;
 	if (skip_prefix(line, "ACK ", &arg)) {
-		if (!get_sha1_hex(arg, result_sha1)) {
+		if (!get_oid_hex(arg, result_oid)) {
 			arg += 40;
 			len -= arg - line;
 			if (len < 1)
@@ -293,7 +293,7 @@ static void send_request(struct fetch_pack_args *args,
 
 static void insert_one_alternate_object(struct object *obj)
 {
-	rev_list_insert_ref(NULL, obj->oid.hash);
+	rev_list_insert_ref(NULL, &obj->oid);
 }
 
 #define INITIAL_FLUSH 16
@@ -317,12 +317,12 @@ static int next_flush(struct fetch_pack_args *args, int count)
 }
 
 static int find_common(struct fetch_pack_args *args,
-		       int fd[2], unsigned char *result_sha1,
+		       int fd[2], struct object_id *result_oid,
 		       struct ref *refs)
 {
 	int fetching;
 	int count = 0, flushes = 0, flush_at = INITIAL_FLUSH, retval;
-	const unsigned char *sha1;
+	const struct object_id *oid;
 	unsigned in_vain = 0;
 	int got_continue = 0;
 	int got_ready = 0;
@@ -340,7 +340,7 @@ static int find_common(struct fetch_pack_args *args,
 
 	fetching = 0;
 	for ( ; refs ; refs = refs->next) {
-		unsigned char *remote = refs->old_oid.hash;
+		struct object_id *remote = &refs->old_oid;
 		const char *remote_hex;
 		struct object *o;
 
@@ -354,12 +354,12 @@ static int find_common(struct fetch_pack_args *args,
 		 * interested in the case we *know* the object is
 		 * reachable and we have already scanned it.
 		 */
-		if (((o = lookup_object(remote)) != NULL) &&
+		if (((o = lookup_object(remote->hash)) != NULL) &&
 				(o->flags & COMPLETE)) {
 			continue;
 		}
 
-		remote_hex = sha1_to_hex(remote);
+		remote_hex = oid_to_hex(remote);
 		if (!fetching) {
 			struct strbuf c = STRBUF_INIT;
 			if (multi_ack == 2)     strbuf_addstr(&c, " multi_ack_detailed");
@@ -410,25 +410,25 @@ static int find_common(struct fetch_pack_args *args,
 	if (args->deepen) {
 		char *line;
 		const char *arg;
-		unsigned char sha1[20];
+		struct object_id oid;
 
 		send_request(args, fd[1], &req_buf);
 		while ((line = packet_read_line(fd[0], NULL))) {
 			if (skip_prefix(line, "shallow ", &arg)) {
-				if (get_sha1_hex(arg, sha1))
+				if (get_oid_hex(arg, &oid))
 					die(_("invalid shallow line: %s"), line);
-				register_shallow(sha1);
+				register_shallow(&oid);
 				continue;
 			}
 			if (skip_prefix(line, "unshallow ", &arg)) {
-				if (get_sha1_hex(arg, sha1))
+				if (get_oid_hex(arg, &oid))
 					die(_("invalid unshallow line: %s"), line);
-				if (!lookup_object(sha1))
+				if (!lookup_object(oid.hash))
 					die(_("object not found: %s"), line);
 				/* make sure that it is parsed as shallow */
-				if (!parse_object(sha1))
+				if (!parse_object(&oid))
 					die(_("error in object: %s"), line);
-				if (unregister_shallow(sha1))
+				if (unregister_shallow(&oid))
 					die(_("no shallow found: %s"), line);
 				continue;
 			}
@@ -447,9 +447,9 @@ static int find_common(struct fetch_pack_args *args,
 
 	flushes = 0;
 	retval = -1;
-	while ((sha1 = get_rev())) {
-		packet_buf_write(&req_buf, "have %s\n", sha1_to_hex(sha1));
-		print_verbose(args, "have %s", sha1_to_hex(sha1));
+	while ((oid = get_rev())) {
+		packet_buf_write(&req_buf, "have %s\n", oid_to_hex(oid));
+		print_verbose(args, "have %s", oid_to_hex(oid));
 		in_vain++;
 		if (flush_at <= ++count) {
 			int ack;
@@ -469,10 +469,10 @@ static int find_common(struct fetch_pack_args *args,
 
 			consume_shallow_list(args, fd[0]);
 			do {
-				ack = get_ack(fd[0], result_sha1);
+				ack = get_ack(fd[0], result_oid);
 				if (ack)
 					print_verbose(args, _("got %s %d %s"), "ack",
-						      ack, sha1_to_hex(result_sha1));
+						      ack, oid_to_hex(result_oid));
 				switch (ack) {
 				case ACK:
 					flushes = 0;
@@ -483,9 +483,9 @@ static int find_common(struct fetch_pack_args *args,
 				case ACK_ready:
 				case ACK_continue: {
 					struct commit *commit =
-						lookup_commit(result_sha1);
+						lookup_commit(result_oid);
 					if (!commit)
-						die(_("invalid commit %s"), sha1_to_hex(result_sha1));
+						die(_("invalid commit %s"), oid_to_hex(result_oid));
 					if (args->stateless_rpc
 					 && ack == ACK_common
 					 && !(commit->object.flags & COMMON)) {
@@ -493,7 +493,7 @@ static int find_common(struct fetch_pack_args *args,
 						 * on the next RPC request so the peer knows
 						 * it is in common with us.
 						 */
-						const char *hex = sha1_to_hex(result_sha1);
+						const char *hex = oid_to_hex(result_oid);
 						packet_buf_write(&req_buf, "have %s\n", hex);
 						state_len = req_buf.len;
 						/*
@@ -538,10 +538,10 @@ done:
 	if (!got_ready || !no_done)
 		consume_shallow_list(args, fd[0]);
 	while (flushes || multi_ack) {
-		int ack = get_ack(fd[0], result_sha1);
+		int ack = get_ack(fd[0], result_oid);
 		if (ack) {
 			print_verbose(args, _("got %s (%d) %s"), "ack",
-				      ack, sha1_to_hex(result_sha1));
+				      ack, oid_to_hex(result_oid));
 			if (ack == ACK)
 				return 0;
 			multi_ack = 1;
@@ -555,16 +555,16 @@ done:
 
 static struct commit_list *complete;
 
-static int mark_complete(const unsigned char *sha1)
+static int mark_complete(const struct object_id *oid)
 {
-	struct object *o = parse_object(sha1);
+	struct object *o = parse_object(oid);
 
 	while (o && o->type == OBJ_TAG) {
 		struct tag *t = (struct tag *) o;
 		if (!t->tagged)
 			break; /* broken repository */
 		o->flags |= COMPLETE;
-		o = parse_object(t->tagged->oid.hash);
+		o = parse_object(&t->tagged->oid);
 	}
 	if (o && o->type == OBJ_COMMIT) {
 		struct commit *commit = (struct commit *)o;
@@ -579,7 +579,7 @@ static int mark_complete(const unsigned char *sha1)
 static int mark_complete_oid(const char *refname, const struct object_id *oid,
 			     int flag, void *cb_data)
 {
-	return mark_complete(oid->hash);
+	return mark_complete(oid);
 }
 
 static void mark_recent_complete_commits(struct fetch_pack_args *args,
@@ -637,14 +637,15 @@ static void filter_refs(struct fetch_pack_args *args,
 
 	/* Append unmatched requests to the list */
 	for (i = 0; i < nr_sought; i++) {
-		unsigned char sha1[20];
+		struct object_id oid;
+		const char *p;
 
 		ref = sought[i];
 		if (ref->match_status != REF_NOT_MATCHED)
 			continue;
-		if (get_sha1_hex(ref->name, sha1) ||
-		    ref->name[40] != '\0' ||
-		    hashcmp(sha1, ref->old_oid.hash))
+		if (parse_oid_hex(ref->name, &oid, &p) ||
+		    *p != '\0' ||
+		    oidcmp(&oid, &ref->old_oid))
 			continue;
 
 		if ((allow_unadvertised_object_request &
@@ -661,7 +662,7 @@ static void filter_refs(struct fetch_pack_args *args,
 
 static void mark_alternate_complete(struct object *obj)
 {
-	mark_complete(obj->oid.hash);
+	mark_complete(&obj->oid);
 }
 
 static int everything_local(struct fetch_pack_args *args,
@@ -680,7 +681,7 @@ static int everything_local(struct fetch_pack_args *args,
 		if (!has_object_file(&ref->old_oid))
 			continue;
 
-		o = parse_object(ref->old_oid.hash);
+		o = parse_object(&ref->old_oid);
 		if (!o)
 			continue;
 
@@ -724,17 +725,17 @@ static int everything_local(struct fetch_pack_args *args,
 	filter_refs(args, refs, sought, nr_sought);
 
 	for (retval = 1, ref = *refs; ref ; ref = ref->next) {
-		const unsigned char *remote = ref->old_oid.hash;
+		const struct object_id *remote = &ref->old_oid;
 		struct object *o;
 
-		o = lookup_object(remote);
+		o = lookup_object(remote->hash);
 		if (!o || !(o->flags & COMPLETE)) {
 			retval = 0;
-			print_verbose(args, "want %s (%s)", sha1_to_hex(remote),
+			print_verbose(args, "want %s (%s)", oid_to_hex(remote),
 				      ref->name);
 			continue;
 		}
-		print_verbose(args, _("already have %s (%s)"), sha1_to_hex(remote),
+		print_verbose(args, _("already have %s (%s)"), oid_to_hex(remote),
 			      ref->name);
 	}
 	return retval;
@@ -873,7 +874,7 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 				 char **pack_lockfile)
 {
 	struct ref *ref = copy_ref_list(orig_ref);
-	unsigned char sha1[20];
+	struct object_id oid;
 	const char *agent_feature;
 	int agent_len;
 
@@ -945,7 +946,7 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 		packet_flush(fd[1]);
 		goto all_done;
 	}
-	if (find_common(args, fd, sha1, ref) < 0)
+	if (find_common(args, fd, &oid, ref) < 0)
 		if (!args->keep_pack)
 			/* When cloning, it is not unusual to have
 			 * no common commit.
diff --git a/fsck.c b/fsck.c
index d589341cdd..b4204d772b 100644
--- a/fsck.c
+++ b/fsck.c
@@ -358,14 +358,14 @@ static int fsck_walk_tree(struct tree *tree, void *data, struct fsck_options *op
 			continue;
 
 		if (S_ISDIR(entry.mode)) {
-			obj = &lookup_tree(entry.oid->hash)->object;
+			obj = &lookup_tree(entry.oid)->object;
 			if (name)
 				put_object_name(options, obj, "%s%s/", name,
 					entry.path);
 			result = options->walk(obj, OBJ_TREE, data, options);
 		}
 		else if (S_ISREG(entry.mode) || S_ISLNK(entry.mode)) {
-			obj = &lookup_blob(entry.oid->hash)->object;
+			obj = &lookup_blob(entry.oid)->object;
 			if (name)
 				put_object_name(options, obj, "%s%s", name,
 					entry.path);
@@ -461,7 +461,7 @@ int fsck_walk(struct object *obj, void *data, struct fsck_options *options)
 		return -1;
 
 	if (obj->type == OBJ_NONE)
-		parse_object(obj->oid.hash);
+		parse_object(&obj->oid);
 
 	switch (obj->type) {
 	case OBJ_BLOB:
diff --git a/http-backend.c b/http-backend.c
index d6ea607533..ba5ff1aa29 100644
--- a/http-backend.c
+++ b/http-backend.c
@@ -431,7 +431,7 @@ static int show_text_ref(const char *name, const struct object_id *oid,
 {
 	const char *name_nons = strip_namespace(name);
 	struct strbuf *buf = cb_data;
-	struct object *o = parse_object(oid->hash);
+	struct object *o = parse_object(oid);
 	if (!o)
 		return 0;
 
diff --git a/http-push.c b/http-push.c
index f0e3108f71..67c4d4b472 100644
--- a/http-push.c
+++ b/http-push.c
@@ -718,13 +718,13 @@ static int fetch_indices(void)
 	return ret;
 }
 
-static void one_remote_object(const unsigned char *sha1)
+static void one_remote_object(const struct object_id *oid)
 {
 	struct object *obj;
 
-	obj = lookup_object(sha1);
+	obj = lookup_object(oid->hash);
 	if (!obj)
-		obj = parse_object(sha1);
+		obj = parse_object(oid);
 
 	/* Ignore remote objects that don't exist locally */
 	if (!obj)
@@ -1013,26 +1013,26 @@ static void remote_ls(const char *path, int flags,
 		      void *userData);
 
 /* extract hex from sharded "xx/x{40}" filename */
-static int get_sha1_hex_from_objpath(const char *path, unsigned char *sha1)
+static int get_oid_hex_from_objpath(const char *path, struct object_id *oid)
 {
-	char hex[40];
+	char hex[GIT_MAX_HEXSZ];
 
-	if (strlen(path) != 41)
+	if (strlen(path) != GIT_SHA1_HEXSZ + 1)
 		return -1;
 
 	memcpy(hex, path, 2);
 	path += 2;
 	path++; /* skip '/' */
-	memcpy(hex, path, 38);
+	memcpy(hex, path, GIT_SHA1_HEXSZ - 2);
 
-	return get_sha1_hex(hex, sha1);
+	return get_oid_hex(hex, oid);
 }
 
 static void process_ls_object(struct remote_ls_ctx *ls)
 {
 	unsigned int *parent = (unsigned int *)ls->userData;
 	const char *path = ls->dentry_name;
-	unsigned char sha1[20];
+	struct object_id oid;
 
 	if (!strcmp(ls->path, ls->dentry_name) && (ls->flags & IS_DIR)) {
 		remote_dir_exists[*parent] = 1;
@@ -1040,10 +1040,10 @@ static void process_ls_object(struct remote_ls_ctx *ls)
 	}
 
 	if (!skip_prefix(path, "objects/", &path) ||
-	    get_sha1_hex_from_objpath(path, sha1))
+	    get_oid_hex_from_objpath(path, &oid))
 		return;
 
-	one_remote_object(sha1);
+	one_remote_object(&oid);
 }
 
 static void process_ls_ref(struct remote_ls_ctx *ls)
@@ -1312,10 +1312,10 @@ static struct object_list **process_tree(struct tree *tree,
 	while (tree_entry(&desc, &entry))
 		switch (object_type(entry.mode)) {
 		case OBJ_TREE:
-			p = process_tree(lookup_tree(entry.oid->hash), p);
+			p = process_tree(lookup_tree(entry.oid), p);
 			break;
 		case OBJ_BLOB:
-			p = process_blob(lookup_blob(entry.oid->hash), p);
+			p = process_blob(lookup_blob(entry.oid), p);
 			break;
 		default:
 			/* Subproject commit - not in this repository */
@@ -1462,7 +1462,7 @@ static void add_remote_info_ref(struct remote_ls_ctx *ls)
 		return;
 	}
 
-	o = parse_object(ref->old_oid.hash);
+	o = parse_object(&ref->old_oid);
 	if (!o) {
 		fprintf(stderr,
 			"Unable to parse object %s for remote ref %s\n",
@@ -1536,7 +1536,7 @@ static int remote_exists(const char *path)
 	return ret;
 }
 
-static void fetch_symref(const char *path, char **symref, unsigned char *sha1)
+static void fetch_symref(const char *path, char **symref, struct object_id *oid)
 {
 	char *url = xstrfmt("%s%s", repo->url, path);
 	struct strbuf buffer = STRBUF_INIT;
@@ -1549,7 +1549,7 @@ static void fetch_symref(const char *path, char **symref, unsigned char *sha1)
 
 	free(*symref);
 	*symref = NULL;
-	hashclr(sha1);
+	oidclr(oid);
 
 	if (buffer.len == 0)
 		return;
@@ -1561,16 +1561,17 @@ static void fetch_symref(const char *path, char **symref, unsigned char *sha1)
 	if (skip_prefix(buffer.buf, "ref: ", &name)) {
 		*symref = xmemdupz(name, buffer.len - (name - buffer.buf));
 	} else {
-		get_sha1_hex(buffer.buf, sha1);
+		get_oid_hex(buffer.buf, oid);
 	}
 
 	strbuf_release(&buffer);
 }
 
-static int verify_merge_base(unsigned char *head_sha1, struct ref *remote)
+static int verify_merge_base(struct object_id *head_oid, struct ref *remote)
 {
-	struct commit *head = lookup_commit_or_die(head_sha1, "HEAD");
-	struct commit *branch = lookup_commit_or_die(remote->old_oid.hash, remote->name);
+	struct commit *head = lookup_commit_or_die(head_oid, "HEAD");
+	struct commit *branch = lookup_commit_or_die(&remote->old_oid,
+						     remote->name);
 
 	return in_merge_bases(branch, head);
 }
@@ -1579,7 +1580,7 @@ static int delete_remote_branch(const char *pattern, int force)
 {
 	struct ref *refs = remote_refs;
 	struct ref *remote_ref = NULL;
-	unsigned char head_sha1[20];
+	struct object_id head_oid;
 	char *symref = NULL;
 	int match;
 	int patlen = strlen(pattern);
@@ -1610,7 +1611,7 @@ static int delete_remote_branch(const char *pattern, int force)
 	 * Remote HEAD must be a symref (not exactly foolproof; a remote
 	 * symlink to a symref will look like a symref)
 	 */
-	fetch_symref("HEAD", &symref, head_sha1);
+	fetch_symref("HEAD", &symref, &head_oid);
 	if (!symref)
 		return error("Remote HEAD is not a symref");
 
@@ -1619,7 +1620,7 @@ static int delete_remote_branch(const char *pattern, int force)
 		if (!strcmp(remote_ref->name, symref))
 			return error("Remote branch %s is the current HEAD",
 				     remote_ref->name);
-		fetch_symref(symref, &symref, head_sha1);
+		fetch_symref(symref, &symref, &head_oid);
 	}
 
 	/* Run extra sanity checks if delete is not forced */
@@ -1627,10 +1628,10 @@ static int delete_remote_branch(const char *pattern, int force)
 		/* Remote HEAD must resolve to a known object */
 		if (symref)
 			return error("Remote HEAD symrefs too deep");
-		if (is_null_sha1(head_sha1))
+		if (is_null_oid(&head_oid))
 			return error("Unable to resolve remote HEAD");
-		if (!has_sha1_file(head_sha1))
-			return error("Remote HEAD resolves to object %s\nwhich does not exist locally, perhaps you need to fetch?", sha1_to_hex(head_sha1));
+		if (!has_object_file(&head_oid))
+			return error("Remote HEAD resolves to object %s\nwhich does not exist locally, perhaps you need to fetch?", oid_to_hex(&head_oid));
 
 		/* Remote branch must resolve to a known object */
 		if (is_null_oid(&remote_ref->old_oid))
@@ -1640,7 +1641,7 @@ static int delete_remote_branch(const char *pattern, int force)
 			return error("Remote branch %s resolves to object %s\nwhich does not exist locally, perhaps you need to fetch?", remote_ref->name, oid_to_hex(&remote_ref->old_oid));
 
 		/* Remote branch must be an ancestor of remote HEAD */
-		if (!verify_merge_base(head_sha1, remote_ref)) {
+		if (!verify_merge_base(&head_oid, remote_ref)) {
 			return error("The branch '%s' is not an ancestor "
 				     "of your current HEAD.\n"
 				     "If you are sure you want to delete it,"
diff --git a/list-objects.c b/list-objects.c
index f3ca6aafb7..b3931fa434 100644
--- a/list-objects.c
+++ b/list-objects.c
@@ -110,7 +110,7 @@ static void process_tree(struct rev_info *revs,
 
 		if (S_ISDIR(entry.mode))
 			process_tree(revs,
-				     lookup_tree(entry.oid->hash),
+				     lookup_tree(entry.oid),
 				     show, base, entry.path,
 				     cb_data);
 		else if (S_ISGITLINK(entry.mode))
@@ -119,7 +119,7 @@ static void process_tree(struct rev_info *revs,
 					cb_data);
 		else
 			process_blob(revs,
-				     lookup_blob(entry.oid->hash),
+				     lookup_blob(entry.oid),
 				     show, base, entry.path,
 				     cb_data);
 	}
diff --git a/log-tree.c b/log-tree.c
index 4618dd04ca..a4ec11c2bf 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -105,13 +105,13 @@ static int add_ref_decoration(const char *refname, const struct object_id *oid,
 			warning("invalid replace ref %s", refname);
 			return 0;
 		}
-		obj = parse_object(original_oid.hash);
+		obj = parse_object(&original_oid);
 		if (obj)
 			add_name_decoration(DECORATION_GRAFTED, "replaced", obj);
 		return 0;
 	}
 
-	obj = parse_object(oid->hash);
+	obj = parse_object(oid);
 	if (!obj)
 		return 0;
 
@@ -132,7 +132,7 @@ static int add_ref_decoration(const char *refname, const struct object_id *oid,
 		if (!obj)
 			break;
 		if (!obj->parsed)
-			parse_object(obj->oid.hash);
+			parse_object(&obj->oid);
 		add_name_decoration(DECORATION_REF_TAG, refname, obj);
 	}
 	return 0;
@@ -140,7 +140,7 @@ static int add_ref_decoration(const char *refname, const struct object_id *oid,
 
 static int add_graft_decoration(const struct commit_graft *graft, void *cb_data)
 {
-	struct commit *commit = lookup_commit(graft->oid.hash);
+	struct commit *commit = lookup_commit(&graft->oid);
 	if (!commit)
 		return 0;
 	add_name_decoration(DECORATION_GRAFTED, "grafted", &commit->object);
@@ -184,7 +184,7 @@ static const struct name_decoration *current_pointed_by_HEAD(const struct name_d
 {
 	const struct name_decoration *list, *head = NULL;
 	const char *branch_name = NULL;
-	unsigned char unused[20];
+	struct object_id unused;
 	int rru_flags;
 
 	/* First find HEAD */
@@ -197,7 +197,7 @@ static const struct name_decoration *current_pointed_by_HEAD(const struct name_d
 		return NULL;
 
 	/* Now resolve and find the matching current branch */
-	branch_name = resolve_ref_unsafe("HEAD", 0, unused, &rru_flags);
+	branch_name = resolve_ref_unsafe("HEAD", 0, unused.hash, &rru_flags);
 	if (!(rru_flags & REF_ISSYMREF))
 		return NULL;
 
@@ -456,13 +456,13 @@ static void show_signature(struct rev_info *opt, struct commit *commit)
 	strbuf_release(&signature);
 }
 
-static int which_parent(const unsigned char *sha1, const struct commit *commit)
+static int which_parent(const struct object_id *oid, const struct commit *commit)
 {
 	int nth;
 	const struct commit_list *parent;
 
 	for (nth = 0, parent = commit->parents; parent; parent = parent->next) {
-		if (!hashcmp(parent->item->object.oid.hash, sha1))
+		if (!oidcmp(&parent->item->object.oid, oid))
 			return nth;
 		nth++;
 	}
@@ -481,14 +481,14 @@ static void show_one_mergetag(struct commit *commit,
 			      void *data)
 {
 	struct rev_info *opt = (struct rev_info *)data;
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct tag *tag;
 	struct strbuf verify_message;
 	int status, nth;
 	size_t payload_size, gpg_message_offset;
 
-	hash_sha1_file(extra->value, extra->len, typename(OBJ_TAG), sha1);
-	tag = lookup_tag(sha1);
+	hash_sha1_file(extra->value, extra->len, typename(OBJ_TAG), oid.hash);
+	tag = lookup_tag(&oid);
 	if (!tag)
 		return; /* error message already given */
 
@@ -500,7 +500,7 @@ static void show_one_mergetag(struct commit *commit,
 			  &commit->parents->next->item->object.oid))
 		strbuf_addf(&verify_message,
 			    "merged tag '%s'\n", tag->tag);
-	else if ((nth = which_parent(tag->tagged->oid.hash, commit)) < 0)
+	else if ((nth = which_parent(&tag->tagged->oid, commit)) < 0)
 		strbuf_addf(&verify_message, "tag %s names a non-parent %s\n",
 				    tag->tag, tag->tagged->oid.hash);
 	else
@@ -536,7 +536,7 @@ void show_log(struct rev_info *opt)
 	struct strbuf msgbuf = STRBUF_INIT;
 	struct log_info *log = opt->loginfo;
 	struct commit *commit = log->commit, *parent = log->parent;
-	int abbrev_commit = opt->abbrev_commit ? opt->abbrev : 40;
+	int abbrev_commit = opt->abbrev_commit ? opt->abbrev : GIT_SHA1_HEXSZ;
 	const char *extra_headers = opt->extra_headers;
 	struct pretty_print_context ctx = {0};
 
diff --git a/merge-recursive.c b/merge-recursive.c
index 62decd51cc..ae5238d82c 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -67,7 +67,7 @@ static struct tree *shift_tree_object(struct tree *one, struct tree *two,
 	}
 	if (!oidcmp(&two->object.oid, &shifted))
 		return two;
-	return lookup_tree(shifted.hash);
+	return lookup_tree(&shifted);
 }
 
 static struct commit *make_virtual_commit(struct tree *tree, const char *comment)
@@ -304,7 +304,7 @@ struct tree *write_tree_from_memory(struct merge_options *o)
 		return NULL;
 	}
 
-	result = lookup_tree(active_cache_tree->sha1);
+	result = lookup_tree(&active_cache_tree->oid);
 
 	return result;
 }
@@ -994,11 +994,11 @@ static int merge_file_1(struct merge_options *o,
 				return ret;
 			result->clean = (merge_status == 0);
 		} else if (S_ISGITLINK(a->mode)) {
-			result->clean = merge_submodule(result->oid.hash,
+			result->clean = merge_submodule(&result->oid,
 						       one->path,
-						       one->oid.hash,
-						       a->oid.hash,
-						       b->oid.hash,
+						       &one->oid,
+						       &a->oid,
+						       &b->oid,
 						       !o->call_depth);
 		} else if (S_ISLNK(a->mode)) {
 			oidcpy(&result->oid, &a->oid);
@@ -2042,7 +2042,7 @@ int merge_recursive(struct merge_options *o,
 		/* if there is no common ancestor, use an empty tree */
 		struct tree *tree;
 
-		tree = lookup_tree(EMPTY_TREE_SHA1_BIN);
+		tree = lookup_tree(&empty_tree_oid);
 		merged_common_ancestors = make_virtual_commit(tree, "ancestor");
 	}
 
@@ -2103,7 +2103,7 @@ static struct commit *get_ref(const struct object_id *oid, const char *name)
 {
 	struct object *object;
 
-	object = deref_tag(parse_object(oid->hash), name, strlen(name));
+	object = deref_tag(parse_object(oid), name, strlen(name));
 	if (!object)
 		return NULL;
 	if (object->type == OBJ_TREE)
diff --git a/merge.c b/merge.c
index 04ee5fc911..1d441ad942 100644
--- a/merge.c
+++ b/merge.c
@@ -44,8 +44,8 @@ int try_merge_command(const char *strategy, size_t xopts_nr,
 	return ret;
 }
 
-int checkout_fast_forward(const unsigned char *head,
-			  const unsigned char *remote,
+int checkout_fast_forward(const struct object_id *head,
+			  const struct object_id *remote,
 			  int overwrite_ignore)
 {
 	struct tree *trees[MAX_UNPACK_TREES];
diff --git a/notes-cache.c b/notes-cache.c
index 5dfc5cbd08..2843e98576 100644
--- a/notes-cache.c
+++ b/notes-cache.c
@@ -5,16 +5,16 @@
 
 static int notes_cache_match_validity(const char *ref, const char *validity)
 {
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct commit *commit;
 	struct pretty_print_context pretty_ctx;
 	struct strbuf msg = STRBUF_INIT;
 	int ret;
 
-	if (read_ref(ref, sha1) < 0)
+	if (read_ref(ref, oid.hash) < 0)
 		return 0;
 
-	commit = lookup_commit_reference_gently(sha1, 1);
+	commit = lookup_commit_reference_gently(&oid, 1);
 	if (!commit)
 		return 0;
 
@@ -46,8 +46,7 @@ void notes_cache_init(struct notes_cache *c, const char *name,
 
 int notes_cache_write(struct notes_cache *c)
 {
-	unsigned char tree_sha1[20];
-	unsigned char commit_sha1[20];
+	struct object_id tree_oid, commit_oid;
 
 	if (!c || !c->tree.initialized || !c->tree.update_ref ||
 	    !*c->tree.update_ref)
@@ -55,19 +54,19 @@ int notes_cache_write(struct notes_cache *c)
 	if (!c->tree.dirty)
 		return 0;
 
-	if (write_notes_tree(&c->tree, tree_sha1))
+	if (write_notes_tree(&c->tree, tree_oid.hash))
 		return -1;
-	if (commit_tree(c->validity, strlen(c->validity), tree_sha1, NULL,
-			commit_sha1, NULL, NULL) < 0)
+	if (commit_tree(c->validity, strlen(c->validity), tree_oid.hash, NULL,
+			commit_oid.hash, NULL, NULL) < 0)
 		return -1;
-	if (update_ref("update notes cache", c->tree.update_ref, commit_sha1,
+	if (update_ref("update notes cache", c->tree.update_ref, commit_oid.hash,
 		       NULL, 0, UPDATE_REFS_QUIET_ON_ERR) < 0)
 		return -1;
 
 	return 0;
 }
 
-char *notes_cache_get(struct notes_cache *c, unsigned char key_sha1[20],
+char *notes_cache_get(struct notes_cache *c, struct object_id *key_oid,
 		      size_t *outsize)
 {
 	const unsigned char *value_sha1;
@@ -75,7 +74,7 @@ char *notes_cache_get(struct notes_cache *c, unsigned char key_sha1[20],
 	char *value;
 	unsigned long size;
 
-	value_sha1 = get_note(&c->tree, key_sha1);
+	value_sha1 = get_note(&c->tree, key_oid->hash);
 	if (!value_sha1)
 		return NULL;
 	value = read_sha1_file(value_sha1, &type, &size);
@@ -84,12 +83,12 @@ char *notes_cache_get(struct notes_cache *c, unsigned char key_sha1[20],
 	return value;
 }
 
-int notes_cache_put(struct notes_cache *c, unsigned char key_sha1[20],
+int notes_cache_put(struct notes_cache *c, struct object_id *key_oid,
 		    const char *data, size_t size)
 {
-	unsigned char value_sha1[20];
+	struct object_id value_oid;
 
-	if (write_sha1_file(data, size, "blob", value_sha1) < 0)
+	if (write_sha1_file(data, size, "blob", value_oid.hash) < 0)
 		return -1;
-	return add_note(&c->tree, key_sha1, value_sha1, NULL);
+	return add_note(&c->tree, key_oid->hash, value_oid.hash, NULL);
 }
diff --git a/notes-cache.h b/notes-cache.h
index 356f88fb3c..aeeee8409d 100644
--- a/notes-cache.h
+++ b/notes-cache.h
@@ -12,9 +12,9 @@ void notes_cache_init(struct notes_cache *c, const char *name,
 		     const char *validity);
 int notes_cache_write(struct notes_cache *c);
 
-char *notes_cache_get(struct notes_cache *c, unsigned char sha1[20], size_t
+char *notes_cache_get(struct notes_cache *c, struct object_id *oid, size_t
 		      *outsize);
-int notes_cache_put(struct notes_cache *c, unsigned char sha1[20],
+int notes_cache_put(struct notes_cache *c, struct object_id *oid,
 		    const char *data, size_t size);
 
 #endif /* NOTES_CACHE_H */
diff --git a/notes-merge.c b/notes-merge.c
index 32caaaff74..6244f6af9c 100644
--- a/notes-merge.c
+++ b/notes-merge.c
@@ -535,7 +535,7 @@ int notes_merge(struct notes_merge_options *o,
 		struct notes_tree *local_tree,
 		unsigned char *result_sha1)
 {
-	unsigned char local_sha1[20], remote_sha1[20];
+	struct object_id local_oid, remote_oid;
 	struct commit *local, *remote;
 	struct commit_list *bases = NULL;
 	const unsigned char *base_sha1, *base_tree_sha1;
@@ -549,46 +549,46 @@ int notes_merge(struct notes_merge_options *o,
 	       o->local_ref, o->remote_ref);
 
 	/* Dereference o->local_ref into local_sha1 */
-	if (read_ref_full(o->local_ref, 0, local_sha1, NULL))
+	if (read_ref_full(o->local_ref, 0, local_oid.hash, NULL))
 		die("Failed to resolve local notes ref '%s'", o->local_ref);
 	else if (!check_refname_format(o->local_ref, 0) &&
-		is_null_sha1(local_sha1))
+		is_null_oid(&local_oid))
 		local = NULL; /* local_sha1 == null_sha1 indicates unborn ref */
-	else if (!(local = lookup_commit_reference(local_sha1)))
+	else if (!(local = lookup_commit_reference(&local_oid)))
 		die("Could not parse local commit %s (%s)",
-		    sha1_to_hex(local_sha1), o->local_ref);
-	trace_printf("\tlocal commit: %.7s\n", sha1_to_hex(local_sha1));
+		    oid_to_hex(&local_oid), o->local_ref);
+	trace_printf("\tlocal commit: %.7s\n", oid_to_hex(&local_oid));
 
 	/* Dereference o->remote_ref into remote_sha1 */
-	if (get_sha1(o->remote_ref, remote_sha1)) {
+	if (get_oid(o->remote_ref, &remote_oid)) {
 		/*
 		 * Failed to get remote_sha1. If o->remote_ref looks like an
 		 * unborn ref, perform the merge using an empty notes tree.
 		 */
 		if (!check_refname_format(o->remote_ref, 0)) {
-			hashclr(remote_sha1);
+			oidclr(&remote_oid);
 			remote = NULL;
 		} else {
 			die("Failed to resolve remote notes ref '%s'",
 			    o->remote_ref);
 		}
-	} else if (!(remote = lookup_commit_reference(remote_sha1))) {
+	} else if (!(remote = lookup_commit_reference(&remote_oid))) {
 		die("Could not parse remote commit %s (%s)",
-		    sha1_to_hex(remote_sha1), o->remote_ref);
+		    oid_to_hex(&remote_oid), o->remote_ref);
 	}
-	trace_printf("\tremote commit: %.7s\n", sha1_to_hex(remote_sha1));
+	trace_printf("\tremote commit: %.7s\n", oid_to_hex(&remote_oid));
 
 	if (!local && !remote)
 		die("Cannot merge empty notes ref (%s) into empty notes ref "
 		    "(%s)", o->remote_ref, o->local_ref);
 	if (!local) {
 		/* result == remote commit */
-		hashcpy(result_sha1, remote_sha1);
+		hashcpy(result_sha1, remote_oid.hash);
 		goto found_result;
 	}
 	if (!remote) {
 		/* result == local commit */
-		hashcpy(result_sha1, local_sha1);
+		hashcpy(result_sha1, local_oid.hash);
 		goto found_result;
 	}
 	assert(local && remote);
diff --git a/notes-utils.c b/notes-utils.c
index 24a33616a4..325ff3daa3 100644
--- a/notes-utils.c
+++ b/notes-utils.c
@@ -7,18 +7,18 @@ void create_notes_commit(struct notes_tree *t, struct commit_list *parents,
 			 const char *msg, size_t msg_len,
 			 unsigned char *result_sha1)
 {
-	unsigned char tree_sha1[20];
+	struct object_id tree_oid;
 
 	assert(t->initialized);
 
-	if (write_notes_tree(t, tree_sha1))
+	if (write_notes_tree(t, tree_oid.hash))
 		die("Failed to write notes tree to database");
 
 	if (!parents) {
 		/* Deduce parent commit from t->ref */
-		unsigned char parent_sha1[20];
-		if (!read_ref(t->ref, parent_sha1)) {
-			struct commit *parent = lookup_commit(parent_sha1);
+		struct object_id parent_oid;
+		if (!read_ref(t->ref, parent_oid.hash)) {
+			struct commit *parent = lookup_commit(&parent_oid);
 			if (parse_commit(parent))
 				die("Failed to find/parse commit %s", t->ref);
 			commit_list_insert(parent, &parents);
@@ -26,14 +26,14 @@ void create_notes_commit(struct notes_tree *t, struct commit_list *parents,
 		/* else: t->ref points to nothing, assume root/orphan commit */
 	}
 
-	if (commit_tree(msg, msg_len, tree_sha1, parents, result_sha1, NULL, NULL))
+	if (commit_tree(msg, msg_len, tree_oid.hash, parents, result_sha1, NULL, NULL))
 		die("Failed to commit notes tree to database");
 }
 
 void commit_notes(struct notes_tree *t, const char *msg)
 {
 	struct strbuf buf = STRBUF_INIT;
-	unsigned char commit_sha1[20];
+	struct object_id commit_oid;
 
 	if (!t)
 		t = &default_notes_tree;
@@ -46,9 +46,9 @@ void commit_notes(struct notes_tree *t, const char *msg)
 	strbuf_addstr(&buf, msg);
 	strbuf_complete_line(&buf);
 
-	create_notes_commit(t, NULL, buf.buf, buf.len, commit_sha1);
+	create_notes_commit(t, NULL, buf.buf, buf.len, commit_oid.hash);
 	strbuf_insert(&buf, 0, "notes: ", 7); /* commit message starts at index 7 */
-	update_ref(buf.buf, t->update_ref, commit_sha1, NULL, 0,
+	update_ref(buf.buf, t->update_ref, commit_oid.hash, NULL, 0,
 		   UPDATE_REFS_DIE_ON_ERR);
 
 	strbuf_release(&buf);
diff --git a/object.c b/object.c
index e680d881a4..06ba3a11d8 100644
--- a/object.c
+++ b/object.c
@@ -180,21 +180,21 @@ struct object *lookup_unknown_object(const unsigned char *sha1)
 	return obj;
 }
 
-struct object *parse_object_buffer(const unsigned char *sha1, enum object_type type, unsigned long size, void *buffer, int *eaten_p)
+struct object *parse_object_buffer(const struct object_id *oid, enum object_type type, unsigned long size, void *buffer, int *eaten_p)
 {
 	struct object *obj;
 	*eaten_p = 0;
 
 	obj = NULL;
 	if (type == OBJ_BLOB) {
-		struct blob *blob = lookup_blob(sha1);
+		struct blob *blob = lookup_blob(oid);
 		if (blob) {
 			if (parse_blob_buffer(blob, buffer, size))
 				return NULL;
 			obj = &blob->object;
 		}
 	} else if (type == OBJ_TREE) {
-		struct tree *tree = lookup_tree(sha1);
+		struct tree *tree = lookup_tree(oid);
 		if (tree) {
 			obj = &tree->object;
 			if (!tree->buffer)
@@ -206,7 +206,7 @@ struct object *parse_object_buffer(const unsigned char *sha1, enum object_type t
 			}
 		}
 	} else if (type == OBJ_COMMIT) {
-		struct commit *commit = lookup_commit(sha1);
+		struct commit *commit = lookup_commit(oid);
 		if (commit) {
 			if (parse_commit_buffer(commit, buffer, size))
 				return NULL;
@@ -217,54 +217,54 @@ struct object *parse_object_buffer(const unsigned char *sha1, enum object_type t
 			obj = &commit->object;
 		}
 	} else if (type == OBJ_TAG) {
-		struct tag *tag = lookup_tag(sha1);
+		struct tag *tag = lookup_tag(oid);
 		if (tag) {
 			if (parse_tag_buffer(tag, buffer, size))
 			       return NULL;
 			obj = &tag->object;
 		}
 	} else {
-		warning("object %s has unknown type id %d", sha1_to_hex(sha1), type);
+		warning("object %s has unknown type id %d", oid_to_hex(oid), type);
 		obj = NULL;
 	}
 	return obj;
 }
 
-struct object *parse_object_or_die(const unsigned char *sha1,
+struct object *parse_object_or_die(const struct object_id *oid,
 				   const char *name)
 {
-	struct object *o = parse_object(sha1);
+	struct object *o = parse_object(oid);
 	if (o)
 		return o;
 
-	die(_("unable to parse object: %s"), name ? name : sha1_to_hex(sha1));
+	die(_("unable to parse object: %s"), name ? name : oid_to_hex(oid));
 }
 
-struct object *parse_object(const unsigned char *sha1)
+struct object *parse_object(const struct object_id *oid)
 {
 	unsigned long size;
 	enum object_type type;
 	int eaten;
-	const unsigned char *repl = lookup_replace_object(sha1);
+	const unsigned char *repl = lookup_replace_object(oid->hash);
 	void *buffer;
 	struct object *obj;
 
-	obj = lookup_object(sha1);
+	obj = lookup_object(oid->hash);
 	if (obj && obj->parsed)
 		return obj;
 
 	if ((obj && obj->type == OBJ_BLOB) ||
-	    (!obj && has_sha1_file(sha1) &&
-	     sha1_object_info(sha1, NULL) == OBJ_BLOB)) {
+	    (!obj && has_object_file(oid) &&
+	     sha1_object_info(oid->hash, NULL) == OBJ_BLOB)) {
 		if (check_sha1_signature(repl, NULL, 0, NULL) < 0) {
-			error("sha1 mismatch %s", sha1_to_hex(repl));
+			error("sha1 mismatch %s", oid_to_hex(oid));
 			return NULL;
 		}
-		parse_blob_buffer(lookup_blob(sha1), NULL, 0);
-		return lookup_object(sha1);
+		parse_blob_buffer(lookup_blob(oid), NULL, 0);
+		return lookup_object(oid->hash);
 	}
 
-	buffer = read_sha1_file(sha1, &type, &size);
+	buffer = read_sha1_file(oid->hash, &type, &size);
 	if (buffer) {
 		if (check_sha1_signature(repl, buffer, size, typename(type)) < 0) {
 			free(buffer);
@@ -272,7 +272,7 @@ struct object *parse_object(const unsigned char *sha1)
 			return NULL;
 		}
 
-		obj = parse_object_buffer(sha1, type, size, buffer, &eaten);
+		obj = parse_object_buffer(oid, type, size, buffer, &eaten);
 		if (!eaten)
 			free(buffer);
 		return obj;
diff --git a/object.h b/object.h
index f52957dcb3..33e5cc9943 100644
--- a/object.h
+++ b/object.h
@@ -89,20 +89,20 @@ void *object_as_type(struct object *obj, enum object_type type, int quiet);
  *
  * Returns NULL if the object is missing or corrupt.
  */
-struct object *parse_object(const unsigned char *sha1);
+struct object *parse_object(const struct object_id *oid);
 
 /*
  * Like parse_object, but will die() instead of returning NULL. If the
  * "name" parameter is not NULL, it is included in the error message
- * (otherwise, the sha1 hex is given).
+ * (otherwise, the hex object ID is given).
  */
-struct object *parse_object_or_die(const unsigned char *sha1, const char *name);
+struct object *parse_object_or_die(const struct object_id *oid, const char *name);
 
 /* Given the result of read_sha1_file(), returns the object after
  * parsing it.  eaten_p indicates if the object has a borrowed copy
  * of buffer and the caller should not free() it.
  */
-struct object *parse_object_buffer(const unsigned char *sha1, enum object_type type, unsigned long size, void *buffer, int *eaten_p);
+struct object *parse_object_buffer(const struct object_id *oid, enum object_type type, unsigned long size, void *buffer, int *eaten_p);
 
 /** Returns the object, with potentially excess memory allocated. **/
 struct object *lookup_unknown_object(const unsigned  char *sha1);
diff --git a/pack-bitmap-write.c b/pack-bitmap-write.c
index e313f4f2bc..8e47a96b3b 100644
--- a/pack-bitmap-write.c
+++ b/pack-bitmap-write.c
@@ -73,7 +73,8 @@ void bitmap_writer_build_type_index(struct pack_idx_entry **index,
 			break;
 
 		default:
-			real_type = sha1_object_info(entry->idx.sha1, NULL);
+			real_type = sha1_object_info(entry->idx.oid.hash,
+						     NULL);
 			break;
 		}
 
@@ -96,7 +97,8 @@ void bitmap_writer_build_type_index(struct pack_idx_entry **index,
 
 		default:
 			die("Missing type information for %s (%d/%d)",
-			    sha1_to_hex(entry->idx.sha1), real_type, entry->type);
+			    oid_to_hex(&entry->idx.oid), real_type,
+			    entry->type);
 		}
 	}
 }
@@ -459,7 +461,7 @@ static inline void dump_bitmap(struct sha1file *f, struct ewah_bitmap *bitmap)
 static const unsigned char *sha1_access(size_t pos, void *table)
 {
 	struct pack_idx_entry **index = table;
-	return index[pos]->sha1;
+	return index[pos]->oid.hash;
 }
 
 static void write_selected_commits_v1(struct sha1file *f,
diff --git a/pack-bitmap.c b/pack-bitmap.c
index 39bcc16846..a3ac3dccd4 100644
--- a/pack-bitmap.c
+++ b/pack-bitmap.c
@@ -673,7 +673,7 @@ int prepare_bitmap_walk(struct rev_info *revs)
 		struct object *object = pending_e[i].item;
 
 		if (object->type == OBJ_NONE)
-			parse_object_or_die(object->oid.hash, NULL);
+			parse_object_or_die(&object->oid, NULL);
 
 		while (object->type == OBJ_TAG) {
 			struct tag *tag = (struct tag *) object;
@@ -685,7 +685,7 @@ int prepare_bitmap_walk(struct rev_info *revs)
 
 			if (!tag->tagged)
 				die("bad tag");
-			object = parse_object_or_die(tag->tagged->oid.hash, NULL);
+			object = parse_object_or_die(&tag->tagged->oid, NULL);
 		}
 
 		if (object->flags & UNINTERESTING)
diff --git a/pack-check.c b/pack-check.c
index 27f70d345f..e1fcb228fa 100644
--- a/pack-check.c
+++ b/pack-check.c
@@ -5,7 +5,10 @@
 
 struct idx_entry {
 	off_t                offset;
-	const unsigned char *sha1;
+	union idx_entry_object {
+		const unsigned char *hash;
+		struct object_id *oid;
+	} oid;
 	unsigned int nr;
 };
 
@@ -51,7 +54,7 @@ static int verify_packfile(struct packed_git *p,
 	off_t index_size = p->index_size;
 	const unsigned char *index_base = p->index_data;
 	git_SHA_CTX ctx;
-	unsigned char sha1[20], *pack_sig;
+	unsigned char hash[GIT_MAX_RAWSZ], *pack_sig;
 	off_t offset = 0, pack_sig_ofs = 0;
 	uint32_t nr_objects, i;
 	int err = 0;
@@ -71,9 +74,9 @@ static int verify_packfile(struct packed_git *p,
 			remaining -= (unsigned int)(offset - pack_sig_ofs);
 		git_SHA1_Update(&ctx, in, remaining);
 	} while (offset < pack_sig_ofs);
-	git_SHA1_Final(sha1, &ctx);
+	git_SHA1_Final(hash, &ctx);
 	pack_sig = use_pack(p, w_curs, pack_sig_ofs, NULL);
-	if (hashcmp(sha1, pack_sig))
+	if (hashcmp(hash, pack_sig))
 		err = error("%s SHA1 checksum mismatch",
 			    p->pack_name);
 	if (hashcmp(index_base + index_size - 40, pack_sig))
@@ -90,8 +93,8 @@ static int verify_packfile(struct packed_git *p,
 	entries[nr_objects].offset = pack_sig_ofs;
 	/* first sort entries by pack offset, since unpacking them is more efficient that way */
 	for (i = 0; i < nr_objects; i++) {
-		entries[i].sha1 = nth_packed_object_sha1(p, i);
-		if (!entries[i].sha1)
+		entries[i].oid.hash = nth_packed_object_sha1(p, i);
+		if (!entries[i].oid.hash)
 			die("internal error pack-check nth-packed-object");
 		entries[i].offset = nth_packed_object_offset(p, i);
 		entries[i].nr = i;
@@ -112,7 +115,7 @@ static int verify_packfile(struct packed_git *p,
 			if (check_pack_crc(p, w_curs, offset, len, nr))
 				err = error("index CRC mismatch for object %s "
 					    "from %s at offset %"PRIuMAX"",
-					    sha1_to_hex(entries[i].sha1),
+					    oid_to_hex(entries[i].oid.oid),
 					    p->pack_name, (uintmax_t)offset);
 		}
 
@@ -135,14 +138,14 @@ static int verify_packfile(struct packed_git *p,
 
 		if (data_valid && !data)
 			err = error("cannot unpack %s from %s at offset %"PRIuMAX"",
-				    sha1_to_hex(entries[i].sha1), p->pack_name,
+				    oid_to_hex(entries[i].oid.oid), p->pack_name,
 				    (uintmax_t)entries[i].offset);
-		else if (check_sha1_signature(entries[i].sha1, data, size, typename(type)))
+		else if (check_sha1_signature(entries[i].oid.hash, data, size, typename(type)))
 			err = error("packed %s from %s is corrupt",
-				    sha1_to_hex(entries[i].sha1), p->pack_name);
+				    oid_to_hex(entries[i].oid.oid), p->pack_name);
 		else if (fn) {
 			int eaten = 0;
-			err |= fn(entries[i].sha1, type, size, data, &eaten);
+			err |= fn(entries[i].oid.oid, type, size, data, &eaten);
 			if (eaten)
 				data = NULL;
 		}
diff --git a/pack-objects.c b/pack-objects.c
index 6398a8aa96..9558d13834 100644
--- a/pack-objects.c
+++ b/pack-objects.c
@@ -14,7 +14,7 @@ static uint32_t locate_object_entry_hash(struct packing_data *pdata,
 	while (pdata->index[i] > 0) {
 		uint32_t pos = pdata->index[i] - 1;
 
-		if (!hashcmp(sha1, pdata->objects[pos].idx.sha1)) {
+		if (!hashcmp(sha1, pdata->objects[pos].idx.oid.hash)) {
 			*found = 1;
 			return i;
 		}
@@ -53,7 +53,9 @@ static void rehash_objects(struct packing_data *pdata)
 
 	for (i = 0; i < pdata->nr_objects; i++) {
 		int found;
-		uint32_t ix = locate_object_entry_hash(pdata, entry->idx.sha1, &found);
+		uint32_t ix = locate_object_entry_hash(pdata,
+						       entry->idx.oid.hash,
+						       &found);
 
 		if (found)
 			die("BUG: Duplicate object in hash");
@@ -98,7 +100,7 @@ struct object_entry *packlist_alloc(struct packing_data *pdata,
 	new_entry = pdata->objects + pdata->nr_objects++;
 
 	memset(new_entry, 0, sizeof(*new_entry));
-	hashcpy(new_entry->idx.sha1, sha1);
+	hashcpy(new_entry->idx.oid.hash, sha1);
 
 	if (pdata->index_size * 3 <= pdata->nr_objects * 4)
 		rehash_objects(pdata);
diff --git a/pack-write.c b/pack-write.c
index fa97b72559..a333ec6754 100644
--- a/pack-write.c
+++ b/pack-write.c
@@ -13,7 +13,7 @@ static int sha1_compare(const void *_a, const void *_b)
 {
 	struct pack_idx_entry *a = *(struct pack_idx_entry **)_a;
 	struct pack_idx_entry *b = *(struct pack_idx_entry **)_b;
-	return hashcmp(a->sha1, b->sha1);
+	return oidcmp(&a->oid, &b->oid);
 }
 
 static int cmp_uint32(const void *a_, const void *b_)
@@ -103,7 +103,7 @@ const char *write_idx_file(const char *index_name, struct pack_idx_entry **objec
 		struct pack_idx_entry **next = list;
 		while (next < last) {
 			struct pack_idx_entry *obj = *next;
-			if (obj->sha1[0] != i)
+			if (obj->oid.hash[0] != i)
 				break;
 			next++;
 		}
@@ -122,11 +122,11 @@ const char *write_idx_file(const char *index_name, struct pack_idx_entry **objec
 			uint32_t offset = htonl(obj->offset);
 			sha1write(f, &offset, 4);
 		}
-		sha1write(f, obj->sha1, 20);
+		sha1write(f, obj->oid.hash, 20);
 		if ((opts->flags & WRITE_IDX_STRICT) &&
-		    (i && !hashcmp(list[-2]->sha1, obj->sha1)))
+		    (i && !oidcmp(&list[-2]->oid, &obj->oid)))
 			die("The same object %s appears twice in the pack",
-			    sha1_to_hex(obj->sha1));
+			    oid_to_hex(&obj->oid));
 	}
 
 	if (index_version >= 2) {
diff --git a/pack.h b/pack.h
index 5c2158746e..8294341af1 100644
--- a/pack.h
+++ b/pack.h
@@ -67,7 +67,7 @@ struct pack_idx_header {
  * Common part of object structure used for write_idx_file
  */
 struct pack_idx_entry {
-	unsigned char sha1[20];
+	struct object_id oid;
 	uint32_t crc32;
 	off_t offset;
 };
@@ -75,7 +75,7 @@ struct pack_idx_entry {
 
 struct progress;
 /* Note, the data argument could be NULL if object type is blob */
-typedef int (*verify_fn)(const unsigned char*, enum object_type, unsigned long, void*, int*);
+typedef int (*verify_fn)(const struct object_id *, enum object_type, unsigned long, void*, int*);
 
 extern const char *write_idx_file(const char *index_name, struct pack_idx_entry **objects, int nr_objects, const struct pack_idx_option *, const unsigned char *sha1);
 extern int check_pack_crc(struct packed_git *p, struct pack_window **w_curs, off_t offset, off_t len, unsigned int nr);
diff --git a/parse-options-cb.c b/parse-options-cb.c
index a6810f295c..c6679cb2cd 100644
--- a/parse-options-cb.c
+++ b/parse-options-cb.c
@@ -80,14 +80,14 @@ int parse_opt_verbosity_cb(const struct option *opt, const char *arg,
 
 int parse_opt_commits(const struct option *opt, const char *arg, int unset)
 {
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct commit *commit;
 
 	if (!arg)
 		return -1;
-	if (get_sha1(arg, sha1))
+	if (get_oid(arg, &oid))
 		return error("malformed object name %s", arg);
-	commit = lookup_commit_reference(sha1);
+	commit = lookup_commit_reference(&oid);
 	if (!commit)
 		return error("no such commit %s", arg);
 	commit_list_insert(commit, opt->value);
diff --git a/pretty.c b/pretty.c
index 587d48371b..09701bd2ff 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1137,7 +1137,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 
 	/* these depend on the commit */
 	if (!commit->object.parsed)
-		parse_object(commit->object.oid.hash);
+		parse_object(&commit->object.oid);
 
 	switch (placeholder[0]) {
 	case 'H':		/* commit hash */
diff --git a/reachable.c b/reachable.c
index 682418f5d2..c62efbfd43 100644
--- a/reachable.c
+++ b/reachable.c
@@ -33,7 +33,7 @@ static int add_one_ref(const char *path, const struct object_id *oid,
 		return 0;
 	}
 
-	object = parse_object_or_die(oid->hash, path);
+	object = parse_object_or_die(oid, path);
 	add_pending_object(revs, object, "");
 
 	return 0;
@@ -82,13 +82,13 @@ static void add_recent_object(const struct object_id *oid,
 	switch (type) {
 	case OBJ_TAG:
 	case OBJ_COMMIT:
-		obj = parse_object_or_die(oid->hash, NULL);
+		obj = parse_object_or_die(oid, NULL);
 		break;
 	case OBJ_TREE:
-		obj = (struct object *)lookup_tree(oid->hash);
+		obj = (struct object *)lookup_tree(oid);
 		break;
 	case OBJ_BLOB:
-		obj = (struct object *)lookup_blob(oid->hash);
+		obj = (struct object *)lookup_blob(oid);
 		break;
 	default:
 		die("unknown object type for %s: %s",
diff --git a/ref-filter.c b/ref-filter.c
index 1fc5e9970d..6cc93dcd9f 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -677,13 +677,13 @@ int verify_ref_format(const char *format)
  * by the "struct object" representation, set *eaten as well---it is a
  * signal from parse_object_buffer to us not to free the buffer.
  */
-static void *get_obj(const unsigned char *sha1, struct object **obj, unsigned long *sz, int *eaten)
+static void *get_obj(const struct object_id *oid, struct object **obj, unsigned long *sz, int *eaten)
 {
 	enum object_type type;
-	void *buf = read_sha1_file(sha1, &type, sz);
+	void *buf = read_sha1_file(oid->hash, &type, sz);
 
 	if (buf)
-		*obj = parse_object_buffer(sha1, type, *sz, buf, eaten);
+		*obj = parse_object_buffer(oid, type, *sz, buf, eaten);
 	else
 		*obj = NULL;
 	return buf;
@@ -1293,7 +1293,7 @@ static void populate_value(struct ref_array_item *ref)
 	struct object *obj;
 	int eaten, i;
 	unsigned long size;
-	const unsigned char *tagged;
+	const struct object_id *tagged;
 
 	ref->value = xcalloc(used_atom_cnt, sizeof(struct atom_value));
 
@@ -1366,14 +1366,14 @@ static void populate_value(struct ref_array_item *ref)
 				v->s = xstrdup(buf + 1);
 			}
 			continue;
-		} else if (!deref && grab_objectname(name, ref->objectname, v, atom)) {
+		} else if (!deref && grab_objectname(name, ref->objectname.hash, v, atom)) {
 			continue;
 		} else if (!strcmp(name, "HEAD")) {
 			const char *head;
-			unsigned char sha1[20];
+			struct object_id oid;
 
 			head = resolve_ref_unsafe("HEAD", RESOLVE_REF_READING,
-						  sha1, NULL);
+						  oid.hash, NULL);
 			if (head && !strcmp(ref->refname, head))
 				v->s = "*";
 			else
@@ -1415,13 +1415,13 @@ static void populate_value(struct ref_array_item *ref)
 	return;
 
  need_obj:
-	buf = get_obj(ref->objectname, &obj, &size, &eaten);
+	buf = get_obj(&ref->objectname, &obj, &size, &eaten);
 	if (!buf)
 		die(_("missing object %s for %s"),
-		    sha1_to_hex(ref->objectname), ref->refname);
+		    oid_to_hex(&ref->objectname), ref->refname);
 	if (!obj)
 		die(_("parse_object_buffer failed on %s for %s"),
-		    sha1_to_hex(ref->objectname), ref->refname);
+		    oid_to_hex(&ref->objectname), ref->refname);
 
 	grab_values(ref->value, 0, obj, buf, size);
 	if (!eaten)
@@ -1438,7 +1438,7 @@ static void populate_value(struct ref_array_item *ref)
 	 * If it is a tag object, see if we use a value that derefs
 	 * the object, and if we do grab the object it refers to.
 	 */
-	tagged = ((struct tag *)obj)->tagged->oid.hash;
+	tagged = &((struct tag *)obj)->tagged->oid;
 
 	/*
 	 * NEEDSWORK: This derefs tag only once, which
@@ -1449,10 +1449,10 @@ static void populate_value(struct ref_array_item *ref)
 	buf = get_obj(tagged, &obj, &size, &eaten);
 	if (!buf)
 		die(_("missing object %s for %s"),
-		    sha1_to_hex(tagged), ref->refname);
+		    oid_to_hex(tagged), ref->refname);
 	if (!obj)
 		die(_("parse_object_buffer failed on %s for %s"),
-		    sha1_to_hex(tagged), ref->refname);
+		    oid_to_hex(tagged), ref->refname);
 	grab_values(ref->value, 1, obj, buf, size);
 	if (!eaten)
 		free(buf);
@@ -1687,7 +1687,7 @@ static const struct object_id *match_points_at(struct oid_array *points_at,
 
 	if (oid_array_lookup(points_at, oid) >= 0)
 		return oid;
-	obj = parse_object(oid->hash);
+	obj = parse_object(oid);
 	if (!obj)
 		die(_("malformed object at '%s'"), refname);
 	if (obj->type == OBJ_TAG)
@@ -1704,7 +1704,7 @@ static struct ref_array_item *new_ref_array_item(const char *refname,
 {
 	struct ref_array_item *ref;
 	FLEX_ALLOC_STR(ref, refname, refname);
-	hashcpy(ref->objectname, objectname);
+	hashcpy(ref->objectname.hash, objectname);
 	ref->flag = flag;
 
 	return ref;
@@ -1782,7 +1782,7 @@ static int ref_filter_handler(const char *refname, const struct object_id *oid,
 	 * non-commits early. The actual filtering is done later.
 	 */
 	if (filter->merge_commit || filter->with_commit || filter->no_commit || filter->verbose) {
-		commit = lookup_commit_reference_gently(oid->hash, 1);
+		commit = lookup_commit_reference_gently(oid, 1);
 		if (!commit)
 			return 0;
 		/* We perform the filtering for the '--contains' option... */
@@ -2090,7 +2090,7 @@ int parse_opt_ref_sorting(const struct option *opt, const char *arg, int unset)
 int parse_opt_merge_filter(const struct option *opt, const char *arg, int unset)
 {
 	struct ref_filter *rf = opt->value;
-	unsigned char sha1[20];
+	struct object_id oid;
 	int no_merged = starts_with(opt->long_name, "no");
 
 	if (rf->merge) {
@@ -2105,10 +2105,10 @@ int parse_opt_merge_filter(const struct option *opt, const char *arg, int unset)
 		? REF_FILTER_MERGED_OMIT
 		: REF_FILTER_MERGED_INCLUDE;
 
-	if (get_sha1(arg, sha1))
+	if (get_oid(arg, &oid))
 		die(_("malformed object name %s"), arg);
 
-	rf->merge_commit = lookup_commit_reference_gently(sha1, 0);
+	rf->merge_commit = lookup_commit_reference_gently(&oid, 0);
 	if (!rf->merge_commit)
 		return opterror(opt, "must point to a commit", 0);
 
diff --git a/ref-filter.h b/ref-filter.h
index c20167aa3c..6552024f09 100644
--- a/ref-filter.h
+++ b/ref-filter.h
@@ -34,7 +34,7 @@ struct ref_sorting {
 };
 
 struct ref_array_item {
-	unsigned char objectname[20];
+	struct object_id objectname;
 	int flag;
 	unsigned int kind;
 	const char *symref;
diff --git a/reflog-walk.c b/reflog-walk.c
index 3ca5ed8415..70905011ee 100644
--- a/reflog-walk.c
+++ b/reflog-walk.c
@@ -238,13 +238,13 @@ void fake_reflog_parent(struct reflog_walk_info *info, struct commit *commit)
 	do {
 		reflog = &commit_reflog->reflogs->items[commit_reflog->recno];
 		commit_reflog->recno--;
-		logobj = parse_object(reflog->ooid.hash);
+		logobj = parse_object(&reflog->ooid);
 	} while (commit_reflog->recno && (logobj && logobj->type != OBJ_COMMIT));
 
-	if (!logobj && commit_reflog->recno >= 0 && is_null_sha1(reflog->ooid.hash)) {
+	if (!logobj && commit_reflog->recno >= 0 && is_null_oid(&reflog->ooid)) {
 		/* a root commit, but there are still more entries to show */
 		reflog = &commit_reflog->reflogs->items[commit_reflog->recno];
-		logobj = parse_object(reflog->noid.hash);
+		logobj = parse_object(&reflog->noid);
 	}
 
 	if (!logobj || logobj->type != OBJ_COMMIT) {
diff --git a/refs.c b/refs.c
index 26d40f9927..8af9641aa1 100644
--- a/refs.c
+++ b/refs.c
@@ -883,9 +883,9 @@ struct ref_update *ref_transaction_add_update(
 	update->flags = flags;
 
 	if (flags & REF_HAVE_NEW)
-		hashcpy(update->new_sha1, new_sha1);
+		hashcpy(update->new_oid.hash, new_sha1);
 	if (flags & REF_HAVE_OLD)
-		hashcpy(update->old_sha1, old_sha1);
+		hashcpy(update->old_oid.hash, old_sha1);
 	update->msg = xstrdup_or_null(msg);
 	return update;
 }
diff --git a/refs.h b/refs.h
index d18ef47128..685a979a0e 100644
--- a/refs.h
+++ b/refs.h
@@ -602,10 +602,10 @@ enum expire_reflog_flags {
  *     unlocked again.
  */
 typedef void reflog_expiry_prepare_fn(const char *refname,
-				      const unsigned char *sha1,
+				      const struct object_id *oid,
 				      void *cb_data);
-typedef int reflog_expiry_should_prune_fn(unsigned char *osha1,
-					  unsigned char *nsha1,
+typedef int reflog_expiry_should_prune_fn(struct object_id *ooid,
+					  struct object_id *noid,
 					  const char *email,
 					  timestamp_t timestamp, int tz,
 					  const char *message, void *cb_data);
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 4925e698d8..cb1f528cbe 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -195,27 +195,15 @@ static const char PACKED_REFS_HEADER[] =
  * Return a pointer to the refname within the line (null-terminated),
  * or NULL if there was a problem.
  */
-static const char *parse_ref_line(struct strbuf *line, unsigned char *sha1)
+static const char *parse_ref_line(struct strbuf *line, struct object_id *oid)
 {
 	const char *ref;
 
-	/*
-	 * 42: the answer to everything.
-	 *
-	 * In this case, it happens to be the answer to
-	 *  40 (length of sha1 hex representation)
-	 *  +1 (space in between hex and name)
-	 *  +1 (newline at the end of the line)
-	 */
-	if (line->len <= 42)
+	if (parse_oid_hex(line->buf, oid, &ref) < 0)
 		return NULL;
-
-	if (get_sha1_hex(line->buf, sha1) < 0)
-		return NULL;
-	if (!isspace(line->buf[40]))
+	if (!isspace(*ref++))
 		return NULL;
 
-	ref = line->buf + 41;
 	if (isspace(*ref))
 		return NULL;
 
@@ -260,7 +248,7 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir)
 	enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled = PEELED_NONE;
 
 	while (strbuf_getwholeline(&line, f, '\n') != EOF) {
-		unsigned char sha1[20];
+		struct object_id oid;
 		const char *refname;
 		const char *traits;
 
@@ -273,17 +261,17 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir)
 			continue;
 		}
 
-		refname = parse_ref_line(&line, sha1);
+		refname = parse_ref_line(&line, &oid);
 		if (refname) {
 			int flag = REF_ISPACKED;
 
 			if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
 				if (!refname_is_safe(refname))
 					die("packed refname is dangerous: %s", refname);
-				hashclr(sha1);
+				oidclr(&oid);
 				flag |= REF_BAD_NAME | REF_ISBROKEN;
 			}
-			last = create_ref_entry(refname, sha1, flag, 0);
+			last = create_ref_entry(refname, &oid, flag, 0);
 			if (peeled == PEELED_FULLY ||
 			    (peeled == PEELED_TAGS && starts_with(refname, "refs/tags/")))
 				last->flag |= REF_KNOWS_PEELED;
@@ -294,8 +282,8 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir)
 		    line.buf[0] == '^' &&
 		    line.len == PEELED_LINE_LENGTH &&
 		    line.buf[PEELED_LINE_LENGTH - 1] == '\n' &&
-		    !get_sha1_hex(line.buf + 1, sha1)) {
-			hashcpy(last->u.value.peeled.hash, sha1);
+		    !get_oid_hex(line.buf + 1, &oid)) {
+			oidcpy(&last->u.value.peeled, &oid);
 			/*
 			 * Regardless of what the file header said,
 			 * we definitely know the value of *this*
@@ -404,14 +392,14 @@ static struct ref_dir *get_packed_refs(struct files_ref_store *refs)
  * commit_packed_refs().
  */
 static void add_packed_ref(struct files_ref_store *refs,
-			   const char *refname, const unsigned char *sha1)
+			   const char *refname, const struct object_id *oid)
 {
 	struct packed_ref_cache *packed_ref_cache = get_packed_ref_cache(refs);
 
 	if (!packed_ref_cache->lock)
 		die("internal error: packed refs not locked");
 	add_ref_entry(get_packed_ref_dir(packed_ref_cache),
-		      create_ref_entry(refname, sha1, REF_ISPACKED, 1));
+		      create_ref_entry(refname, oid, REF_ISPACKED, 1));
 }
 
 /*
@@ -444,7 +432,7 @@ static void loose_fill_ref_dir(struct ref_store *ref_store,
 	strbuf_add(&refname, dirname, dirnamelen);
 
 	while ((de = readdir(d)) != NULL) {
-		unsigned char sha1[20];
+		struct object_id oid;
 		struct stat st;
 		int flag;
 
@@ -465,10 +453,10 @@ static void loose_fill_ref_dir(struct ref_store *ref_store,
 			if (!refs_resolve_ref_unsafe(&refs->base,
 						     refname.buf,
 						     RESOLVE_REF_READING,
-						     sha1, &flag)) {
-				hashclr(sha1);
+						     oid.hash, &flag)) {
+				oidclr(&oid);
 				flag |= REF_ISBROKEN;
-			} else if (is_null_sha1(sha1)) {
+			} else if (is_null_oid(&oid)) {
 				/*
 				 * It is so astronomically unlikely
 				 * that NULL_SHA1 is the SHA-1 of an
@@ -484,11 +472,11 @@ static void loose_fill_ref_dir(struct ref_store *ref_store,
 						 REFNAME_ALLOW_ONELEVEL)) {
 				if (!refname_is_safe(refname.buf))
 					die("loose refname is dangerous: %s", refname.buf);
-				hashclr(sha1);
+				oidclr(&oid);
 				flag |= REF_BAD_NAME | REF_ISBROKEN;
 			}
 			add_entry_to_dir(dir,
-					 create_ref_entry(refname.buf, sha1, flag, 0));
+					 create_ref_entry(refname.buf, &oid, flag, 0));
 		}
 		strbuf_setlen(&refname, dirnamelen);
 		strbuf_setlen(&path, path_baselen);
@@ -1526,7 +1514,7 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 			packed_entry->flag = REF_ISPACKED;
 			oidcpy(&packed_entry->u.value.oid, iter->oid);
 		} else {
-			packed_entry = create_ref_entry(iter->refname, iter->oid->hash,
+			packed_entry = create_ref_entry(iter->refname, iter->oid,
 							REF_ISPACKED, 0);
 			add_ref_entry(packed_refs, packed_entry);
 		}
@@ -1709,10 +1697,10 @@ static int rename_tmp_log(struct files_ref_store *refs, const char *newrefname)
 }
 
 static int write_ref_to_lockfile(struct ref_lock *lock,
-				 const unsigned char *sha1, struct strbuf *err);
+				 const struct object_id *oid, struct strbuf *err);
 static int commit_ref_update(struct files_ref_store *refs,
 			     struct ref_lock *lock,
-			     const unsigned char *sha1, const char *logmsg,
+			     const struct object_id *oid, const char *logmsg,
 			     struct strbuf *err);
 
 static int files_rename_ref(struct ref_store *ref_store,
@@ -1721,7 +1709,7 @@ static int files_rename_ref(struct ref_store *ref_store,
 {
 	struct files_ref_store *refs =
 		files_downcast(ref_store, REF_STORE_WRITE, "rename_ref");
-	unsigned char sha1[20], orig_sha1[20];
+	struct object_id oid, orig_oid;
 	int flag = 0, logmoved = 0;
 	struct ref_lock *lock;
 	struct stat loginfo;
@@ -1743,7 +1731,7 @@ static int files_rename_ref(struct ref_store *ref_store,
 
 	if (!refs_resolve_ref_unsafe(&refs->base, oldrefname,
 				     RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
-				orig_sha1, &flag)) {
+				orig_oid.hash, &flag)) {
 		ret = error("refname %s not found", oldrefname);
 		goto out;
 	}
@@ -1765,21 +1753,21 @@ static int files_rename_ref(struct ref_store *ref_store,
 	}
 
 	if (refs_delete_ref(&refs->base, logmsg, oldrefname,
-			    orig_sha1, REF_NODEREF)) {
+			    orig_oid.hash, REF_NODEREF)) {
 		error("unable to delete old %s", oldrefname);
 		goto rollback;
 	}
 
 	/*
-	 * Since we are doing a shallow lookup, sha1 is not the
-	 * correct value to pass to delete_ref as old_sha1. But that
-	 * doesn't matter, because an old_sha1 check wouldn't add to
+	 * Since we are doing a shallow lookup, oid is not the
+	 * correct value to pass to delete_ref as old_oid. But that
+	 * doesn't matter, because an old_oid check wouldn't add to
 	 * the safety anyway; we want to delete the reference whatever
 	 * its current value.
 	 */
 	if (!refs_read_ref_full(&refs->base, newrefname,
 				RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
-				sha1, NULL) &&
+				oid.hash, NULL) &&
 	    refs_delete_ref(&refs->base, NULL, newrefname,
 			    NULL, REF_NODEREF)) {
 		if (errno == EISDIR) {
@@ -1812,10 +1800,10 @@ static int files_rename_ref(struct ref_store *ref_store,
 		strbuf_release(&err);
 		goto rollback;
 	}
-	hashcpy(lock->old_oid.hash, orig_sha1);
+	oidcpy(&lock->old_oid, &orig_oid);
 
-	if (write_ref_to_lockfile(lock, orig_sha1, &err) ||
-	    commit_ref_update(refs, lock, orig_sha1, logmsg, &err)) {
+	if (write_ref_to_lockfile(lock, &orig_oid, &err) ||
+	    commit_ref_update(refs, lock, &orig_oid, logmsg, &err)) {
 		error("unable to write current sha1 into %s: %s", newrefname, err.buf);
 		strbuf_release(&err);
 		goto rollback;
@@ -1835,8 +1823,8 @@ static int files_rename_ref(struct ref_store *ref_store,
 
 	flag = log_all_ref_updates;
 	log_all_ref_updates = LOG_REFS_NONE;
-	if (write_ref_to_lockfile(lock, orig_sha1, &err) ||
-	    commit_ref_update(refs, lock, orig_sha1, NULL, &err)) {
+	if (write_ref_to_lockfile(lock, &orig_oid, &err) ||
+	    commit_ref_update(refs, lock, &orig_oid, NULL, &err)) {
 		error("unable to write current sha1 into %s: %s", oldrefname, err.buf);
 		strbuf_release(&err);
 	}
@@ -1986,8 +1974,8 @@ static int files_create_reflog(struct ref_store *ref_store,
 	return 0;
 }
 
-static int log_ref_write_fd(int fd, const unsigned char *old_sha1,
-			    const unsigned char *new_sha1,
+static int log_ref_write_fd(int fd, const struct object_id *old_oid,
+			    const struct object_id *new_oid,
 			    const char *committer, const char *msg)
 {
 	int msglen, written;
@@ -1998,8 +1986,8 @@ static int log_ref_write_fd(int fd, const unsigned char *old_sha1,
 	maxlen = strlen(committer) + msglen + 100;
 	logrec = xmalloc(maxlen);
 	len = xsnprintf(logrec, maxlen, "%s %s %s\n",
-			sha1_to_hex(old_sha1),
-			sha1_to_hex(new_sha1),
+			oid_to_hex(old_oid),
+			oid_to_hex(new_oid),
 			committer);
 	if (msglen)
 		len += copy_reflog_msg(logrec + len - 1, msg) - 1;
@@ -2013,8 +2001,8 @@ static int log_ref_write_fd(int fd, const unsigned char *old_sha1,
 }
 
 static int files_log_ref_write(struct files_ref_store *refs,
-			       const char *refname, const unsigned char *old_sha1,
-			       const unsigned char *new_sha1, const char *msg,
+			       const char *refname, const struct object_id *old_oid,
+			       const struct object_id *new_oid, const char *msg,
 			       int flags, struct strbuf *err)
 {
 	int logfd, result;
@@ -2031,7 +2019,7 @@ static int files_log_ref_write(struct files_ref_store *refs,
 
 	if (logfd < 0)
 		return 0;
-	result = log_ref_write_fd(logfd, old_sha1, new_sha1,
+	result = log_ref_write_fd(logfd, old_oid, new_oid,
 				  git_committer_info(0), msg);
 	if (result) {
 		struct strbuf sb = STRBUF_INIT;
@@ -2063,29 +2051,29 @@ static int files_log_ref_write(struct files_ref_store *refs,
  * return -1.
  */
 static int write_ref_to_lockfile(struct ref_lock *lock,
-				 const unsigned char *sha1, struct strbuf *err)
+				 const struct object_id *oid, struct strbuf *err)
 {
 	static char term = '\n';
 	struct object *o;
 	int fd;
 
-	o = parse_object(sha1);
+	o = parse_object(oid);
 	if (!o) {
 		strbuf_addf(err,
 			    "trying to write ref '%s' with nonexistent object %s",
-			    lock->ref_name, sha1_to_hex(sha1));
+			    lock->ref_name, oid_to_hex(oid));
 		unlock_ref(lock);
 		return -1;
 	}
 	if (o->type != OBJ_COMMIT && is_branch(lock->ref_name)) {
 		strbuf_addf(err,
 			    "trying to write non-commit object %s to branch '%s'",
-			    sha1_to_hex(sha1), lock->ref_name);
+			    oid_to_hex(oid), lock->ref_name);
 		unlock_ref(lock);
 		return -1;
 	}
 	fd = get_lock_file_fd(lock->lk);
-	if (write_in_full(fd, sha1_to_hex(sha1), 40) != 40 ||
+	if (write_in_full(fd, oid_to_hex(oid), GIT_SHA1_HEXSZ) != GIT_SHA1_HEXSZ ||
 	    write_in_full(fd, &term, 1) != 1 ||
 	    close_ref(lock) < 0) {
 		strbuf_addf(err,
@@ -2103,14 +2091,14 @@ static int write_ref_to_lockfile(struct ref_lock *lock,
  */
 static int commit_ref_update(struct files_ref_store *refs,
 			     struct ref_lock *lock,
-			     const unsigned char *sha1, const char *logmsg,
+			     const struct object_id *oid, const char *logmsg,
 			     struct strbuf *err)
 {
 	files_assert_main_repository(refs, "commit_ref_update");
 
 	clear_loose_ref_cache(refs);
 	if (files_log_ref_write(refs, lock->ref_name,
-				lock->old_oid.hash, sha1,
+				&lock->old_oid, oid,
 				logmsg, 0, err)) {
 		char *old_msg = strbuf_detach(err, NULL);
 		strbuf_addf(err, "cannot update the ref '%s': %s",
@@ -2133,18 +2121,18 @@ static int commit_ref_update(struct files_ref_store *refs,
 		 * check with HEAD only which should cover 99% of all usage
 		 * scenarios (even 100% of the default ones).
 		 */
-		unsigned char head_sha1[20];
+		struct object_id head_oid;
 		int head_flag;
 		const char *head_ref;
 
 		head_ref = refs_resolve_ref_unsafe(&refs->base, "HEAD",
 						   RESOLVE_REF_READING,
-						   head_sha1, &head_flag);
+						   head_oid.hash, &head_flag);
 		if (head_ref && (head_flag & REF_ISSYMREF) &&
 		    !strcmp(head_ref, lock->ref_name)) {
 			struct strbuf log_err = STRBUF_INIT;
 			if (files_log_ref_write(refs, "HEAD",
-						lock->old_oid.hash, sha1,
+						&lock->old_oid, oid,
 						logmsg, 0, &log_err)) {
 				error("%s", log_err.buf);
 				strbuf_release(&log_err);
@@ -2182,12 +2170,12 @@ static void update_symref_reflog(struct files_ref_store *refs,
 				 const char *target, const char *logmsg)
 {
 	struct strbuf err = STRBUF_INIT;
-	unsigned char new_sha1[20];
+	struct object_id new_oid;
 	if (logmsg &&
 	    !refs_read_ref_full(&refs->base, target,
-				RESOLVE_REF_READING, new_sha1, NULL) &&
-	    files_log_ref_write(refs, refname, lock->old_oid.hash,
-				new_sha1, logmsg, 0, &err)) {
+				RESOLVE_REF_READING, new_oid.hash, NULL) &&
+	    files_log_ref_write(refs, refname, &lock->old_oid,
+				&new_oid, logmsg, 0, &err)) {
 		error("%s", err.buf);
 		strbuf_release(&err);
 	}
@@ -2589,7 +2577,7 @@ static int split_head_update(struct ref_update *update,
 	new_update = ref_transaction_add_update(
 			transaction, "HEAD",
 			update->flags | REF_LOG_ONLY | REF_NODEREF,
-			update->new_sha1, update->old_sha1,
+			update->new_oid.hash, update->old_oid.hash,
 			update->msg);
 
 	item->util = new_update;
@@ -2646,7 +2634,7 @@ static int split_symref_update(struct files_ref_store *refs,
 
 	new_update = ref_transaction_add_update(
 			transaction, referent, new_flags,
-			update->new_sha1, update->old_sha1,
+			update->new_oid.hash, update->old_oid.hash,
 			update->msg);
 
 	new_update->parent_update = update;
@@ -2685,10 +2673,10 @@ static int check_old_oid(struct ref_update *update, struct object_id *oid,
 			 struct strbuf *err)
 {
 	if (!(update->flags & REF_HAVE_OLD) ||
-		   !hashcmp(oid->hash, update->old_sha1))
+		   !oidcmp(oid, &update->old_oid))
 		return 0;
 
-	if (is_null_sha1(update->old_sha1))
+	if (is_null_oid(&update->old_oid))
 		strbuf_addf(err, "cannot lock ref '%s': "
 			    "reference already exists",
 			    original_update_refname(update));
@@ -2696,13 +2684,13 @@ static int check_old_oid(struct ref_update *update, struct object_id *oid,
 		strbuf_addf(err, "cannot lock ref '%s': "
 			    "reference is missing but expected %s",
 			    original_update_refname(update),
-			    sha1_to_hex(update->old_sha1));
+			    oid_to_hex(&update->old_oid));
 	else
 		strbuf_addf(err, "cannot lock ref '%s': "
 			    "is at %s but expected %s",
 			    original_update_refname(update),
 			    oid_to_hex(oid),
-			    sha1_to_hex(update->old_sha1));
+			    oid_to_hex(&update->old_oid));
 
 	return -1;
 }
@@ -2729,13 +2717,13 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 {
 	struct strbuf referent = STRBUF_INIT;
 	int mustexist = (update->flags & REF_HAVE_OLD) &&
-		!is_null_sha1(update->old_sha1);
+		!is_null_oid(&update->old_oid);
 	int ret;
 	struct ref_lock *lock;
 
 	files_assert_main_repository(refs, "lock_ref_for_update");
 
-	if ((update->flags & REF_HAVE_NEW) && is_null_sha1(update->new_sha1))
+	if ((update->flags & REF_HAVE_NEW) && is_null_oid(&update->new_oid))
 		update->flags |= REF_DELETING;
 
 	if (head_ref) {
@@ -2817,12 +2805,12 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 	    !(update->flags & REF_DELETING) &&
 	    !(update->flags & REF_LOG_ONLY)) {
 		if (!(update->type & REF_ISSYMREF) &&
-		    !hashcmp(lock->old_oid.hash, update->new_sha1)) {
+		    !oidcmp(&lock->old_oid, &update->new_oid)) {
 			/*
 			 * The reference already has the desired
 			 * value, so we don't need to write it.
 			 */
-		} else if (write_ref_to_lockfile(lock, update->new_sha1,
+		} else if (write_ref_to_lockfile(lock, &update->new_oid,
 						 err)) {
 			char *write_err = strbuf_detach(err, NULL);
 
@@ -2957,8 +2945,8 @@ static int files_transaction_commit(struct ref_store *ref_store,
 		    update->flags & REF_LOG_ONLY) {
 			if (files_log_ref_write(refs,
 						lock->ref_name,
-						lock->old_oid.hash,
-						update->new_sha1,
+						&lock->old_oid,
+						&update->new_oid,
 						update->msg, update->flags,
 						err)) {
 				char *old_msg = strbuf_detach(err, NULL);
@@ -3107,7 +3095,7 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 		struct ref_update *update = transaction->updates[i];
 
 		if ((update->flags & REF_HAVE_OLD) &&
-		    !is_null_sha1(update->old_sha1))
+		    !is_null_oid(&update->old_oid))
 			die("BUG: initial ref transaction with old_sha1 set");
 		if (refs_verify_refname_available(&refs->base, update->refname,
 						  &affected_refnames, NULL,
@@ -3128,8 +3116,9 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 		struct ref_update *update = transaction->updates[i];
 
 		if ((update->flags & REF_HAVE_NEW) &&
-		    !is_null_sha1(update->new_sha1))
-			add_packed_ref(refs, update->refname, update->new_sha1);
+		    !is_null_oid(&update->new_oid))
+			add_packed_ref(refs, update->refname,
+				       &update->new_oid);
 	}
 
 	if (commit_packed_refs(refs)) {
@@ -3163,7 +3152,7 @@ static int expire_reflog_ent(struct object_id *ooid, struct object_id *noid,
 	if (cb->flags & EXPIRE_REFLOGS_REWRITE)
 		ooid = &cb->last_kept_oid;
 
-	if ((*cb->should_prune_fn)(ooid->hash, noid->hash, email, timestamp, tz,
+	if ((*cb->should_prune_fn)(ooid, noid, email, timestamp, tz,
 				   message, policy_cb)) {
 		if (!cb->newlog)
 			printf("would prune %s", message);
@@ -3200,6 +3189,7 @@ static int files_reflog_expire(struct ref_store *ref_store,
 	int status = 0;
 	int type;
 	struct strbuf err = STRBUF_INIT;
+	struct object_id oid;
 
 	memset(&cb, 0, sizeof(cb));
 	cb.flags = flags;
@@ -3249,7 +3239,9 @@ static int files_reflog_expire(struct ref_store *ref_store,
 		}
 	}
 
-	(*prepare_fn)(refname, sha1, cb.policy_cb);
+	hashcpy(oid.hash, sha1);
+
+	(*prepare_fn)(refname, &oid, cb.policy_cb);
 	refs_for_each_reflog_ent(ref_store, refname, expire_reflog_ent, &cb);
 	(*cleanup_fn)(cb.policy_cb);
 
diff --git a/refs/ref-cache.c b/refs/ref-cache.c
index 6059362f1d..6b11d9cd12 100644
--- a/refs/ref-cache.c
+++ b/refs/ref-cache.c
@@ -32,7 +32,7 @@ struct ref_dir *get_ref_dir(struct ref_entry *entry)
 }
 
 struct ref_entry *create_ref_entry(const char *refname,
-				   const unsigned char *sha1, int flag,
+				   const struct object_id *oid, int flag,
 				   int check_name)
 {
 	struct ref_entry *ref;
@@ -41,7 +41,7 @@ struct ref_entry *create_ref_entry(const char *refname,
 	    check_refname_format(refname, REFNAME_ALLOW_ONELEVEL))
 		die("Reference has invalid format: '%s'", refname);
 	FLEX_ALLOC_STR(ref, name, refname);
-	hashcpy(ref->u.value.oid.hash, sha1);
+	oidcpy(&ref->u.value.oid, oid);
 	oidclr(&ref->u.value.peeled);
 	ref->flag = flag;
 	return ref;
diff --git a/refs/ref-cache.h b/refs/ref-cache.h
index ffdc54f3f0..1f65e2f9ed 100644
--- a/refs/ref-cache.h
+++ b/refs/ref-cache.h
@@ -185,7 +185,7 @@ struct ref_entry *create_dir_entry(struct ref_cache *cache,
 				   int incomplete);
 
 struct ref_entry *create_ref_entry(const char *refname,
-				   const unsigned char *sha1, int flag,
+				   const struct object_id *oid, int flag,
 				   int check_name);
 
 /*
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 12cf4e4718..b6b291cf00 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -130,13 +130,13 @@ struct ref_update {
 	/*
 	 * If (flags & REF_HAVE_NEW), set the reference to this value:
 	 */
-	unsigned char new_sha1[20];
+	struct object_id new_oid;
 
 	/*
 	 * If (flags & REF_HAVE_OLD), check that the reference
 	 * previously had this value:
 	 */
-	unsigned char old_sha1[20];
+	struct object_id old_oid;
 
 	/*
 	 * One or more of REF_HAVE_NEW, REF_HAVE_OLD, REF_NODEREF,
diff --git a/remote.c b/remote.c
index 801137c72e..38ca1353b9 100644
--- a/remote.c
+++ b/remote.c
@@ -1296,7 +1296,7 @@ static void add_to_tips(struct tips *tips, const struct object_id *oid)
 
 	if (is_null_oid(oid))
 		return;
-	commit = lookup_commit_reference_gently(oid->hash, 1);
+	commit = lookup_commit_reference_gently(oid, 1);
 	if (!commit || (commit->object.flags & TMP_MARK))
 		return;
 	commit->object.flags |= TMP_MARK;
@@ -1358,7 +1358,8 @@ static void add_missing_tags(struct ref *src, struct ref **dst, struct ref ***ds
 
 			if (is_null_oid(&ref->new_oid))
 				continue;
-			commit = lookup_commit_reference_gently(ref->new_oid.hash, 1);
+			commit = lookup_commit_reference_gently(&ref->new_oid,
+								1);
 			if (!commit)
 				/* not pushing a commit, which is not an error */
 				continue;
@@ -1585,8 +1586,8 @@ void set_ref_status_for_push(struct ref *remote_refs, int send_mirror,
 				reject_reason = REF_STATUS_REJECT_ALREADY_EXISTS;
 			else if (!has_object_file(&ref->old_oid))
 				reject_reason = REF_STATUS_REJECT_FETCH_FIRST;
-			else if (!lookup_commit_reference_gently(ref->old_oid.hash, 1) ||
-				 !lookup_commit_reference_gently(ref->new_oid.hash, 1))
+			else if (!lookup_commit_reference_gently(&ref->old_oid, 1) ||
+				 !lookup_commit_reference_gently(&ref->new_oid, 1))
 				reject_reason = REF_STATUS_REJECT_NEEDS_FORCE;
 			else if (!ref_newer(&ref->new_oid, &ref->old_oid))
 				reject_reason = REF_STATUS_REJECT_NONFASTFORWARD;
@@ -1953,12 +1954,12 @@ int ref_newer(const struct object_id *new_oid, const struct object_id *old_oid)
 	 * Both new and old must be commit-ish and new is descendant of
 	 * old.  Otherwise we require --force.
 	 */
-	o = deref_tag(parse_object(old_oid->hash), NULL, 0);
+	o = deref_tag(parse_object(old_oid), NULL, 0);
 	if (!o || o->type != OBJ_COMMIT)
 		return 0;
 	old = (struct commit *) o;
 
-	o = deref_tag(parse_object(new_oid->hash), NULL, 0);
+	o = deref_tag(parse_object(new_oid), NULL, 0);
 	if (!o || o->type != OBJ_COMMIT)
 		return 0;
 	new = (struct commit *) o;
@@ -2009,13 +2010,13 @@ int stat_tracking_info(struct branch *branch, int *num_ours, int *num_theirs,
 	/* Cannot stat if what we used to build on no longer exists */
 	if (read_ref(base, oid.hash))
 		return -1;
-	theirs = lookup_commit_reference(oid.hash);
+	theirs = lookup_commit_reference(&oid);
 	if (!theirs)
 		return -1;
 
 	if (read_ref(branch->refname, oid.hash))
 		return -1;
-	ours = lookup_commit_reference(oid.hash);
+	ours = lookup_commit_reference(&oid);
 	if (!ours)
 		return -1;
 
diff --git a/revision.c b/revision.c
index 8a8c1789c7..9c67cb6026 100644
--- a/revision.c
+++ b/revision.c
@@ -59,10 +59,10 @@ static void mark_tree_contents_uninteresting(struct tree *tree)
 	while (tree_entry(&desc, &entry)) {
 		switch (object_type(entry.mode)) {
 		case OBJ_TREE:
-			mark_tree_uninteresting(lookup_tree(entry.oid->hash));
+			mark_tree_uninteresting(lookup_tree(entry.oid));
 			break;
 		case OBJ_BLOB:
-			mark_blob_uninteresting(lookup_blob(entry.oid->hash));
+			mark_blob_uninteresting(lookup_blob(entry.oid));
 			break;
 		default:
 			/* Subproject commit - not in this repository */
@@ -177,23 +177,23 @@ void add_pending_object(struct rev_info *revs,
 
 void add_head_to_pending(struct rev_info *revs)
 {
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct object *obj;
-	if (get_sha1("HEAD", sha1))
+	if (get_oid("HEAD", &oid))
 		return;
-	obj = parse_object(sha1);
+	obj = parse_object(&oid);
 	if (!obj)
 		return;
 	add_pending_object(revs, obj, "HEAD");
 }
 
 static struct object *get_reference(struct rev_info *revs, const char *name,
-				    const unsigned char *sha1,
+				    const struct object_id *oid,
 				    unsigned int flags)
 {
 	struct object *object;
 
-	object = parse_object(sha1);
+	object = parse_object(oid);
 	if (!object) {
 		if (revs->ignore_missing)
 			return object;
@@ -203,10 +203,10 @@ static struct object *get_reference(struct rev_info *revs, const char *name,
 	return object;
 }
 
-void add_pending_sha1(struct rev_info *revs, const char *name,
-		      const unsigned char *sha1, unsigned int flags)
+void add_pending_oid(struct rev_info *revs, const char *name,
+		      const struct object_id *oid, unsigned int flags)
 {
-	struct object *object = get_reference(revs, name, sha1, flags);
+	struct object *object = get_reference(revs, name, oid, flags);
 	add_pending_object(revs, object, name);
 }
 
@@ -228,7 +228,7 @@ static struct commit *handle_commit(struct rev_info *revs,
 			add_pending_object(revs, object, tag->tag);
 		if (!tag->tagged)
 			die("bad tag");
-		object = parse_object(tag->tagged->oid.hash);
+		object = parse_object(&tag->tagged->oid);
 		if (!object) {
 			if (flags & UNINTERESTING)
 				return NULL;
@@ -1157,9 +1157,9 @@ static int handle_one_ref(const char *path, const struct object_id *oid,
 	if (ref_excluded(cb->all_revs->ref_excludes, path))
 	    return 0;
 
-	object = get_reference(cb->all_revs, path, oid->hash, cb->all_flags);
+	object = get_reference(cb->all_revs, path, oid, cb->all_flags);
 	add_rev_cmdline(cb->all_revs, object, path, REV_CMD_REF, cb->all_flags);
-	add_pending_sha1(cb->all_revs, path, oid->hash, cb->all_flags);
+	add_pending_oid(cb->all_revs, path, oid, cb->all_flags);
 	return 0;
 }
 
@@ -1200,7 +1200,7 @@ static void handle_one_reflog_commit(struct object_id *oid, void *cb_data)
 {
 	struct all_refs_cb *cb = cb_data;
 	if (!is_null_oid(oid)) {
-		struct object *o = parse_object(oid->hash);
+		struct object *o = parse_object(oid);
 		if (o) {
 			o->flags |= cb->all_flags;
 			/* ??? CMDLINEFLAGS ??? */
@@ -1249,7 +1249,7 @@ static void add_cache_tree(struct cache_tree *it, struct rev_info *revs,
 	int i;
 
 	if (it->entry_count >= 0) {
-		struct tree *tree = lookup_tree(it->sha1);
+		struct tree *tree = lookup_tree(&it->oid);
 		add_pending_object_with_path(revs, &tree->object, "",
 					     040000, path->buf);
 	}
@@ -1275,7 +1275,7 @@ void add_index_objects_to_pending(struct rev_info *revs, unsigned flags)
 		if (S_ISGITLINK(ce->ce_mode))
 			continue;
 
-		blob = lookup_blob(ce->oid.hash);
+		blob = lookup_blob(&ce->oid);
 		if (!blob)
 			die("unable to add index blob to traversal");
 		add_pending_object_with_path(revs, &blob->object, "",
@@ -1292,7 +1292,7 @@ void add_index_objects_to_pending(struct rev_info *revs, unsigned flags)
 static int add_parents_only(struct rev_info *revs, const char *arg_, int flags,
 			    int exclude_parent)
 {
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct object *it;
 	struct commit *commit;
 	struct commit_list *parents;
@@ -1303,17 +1303,17 @@ static int add_parents_only(struct rev_info *revs, const char *arg_, int flags,
 		flags ^= UNINTERESTING | BOTTOM;
 		arg++;
 	}
-	if (get_sha1_committish(arg, sha1))
+	if (get_sha1_committish(arg, oid.hash))
 		return 0;
 	while (1) {
-		it = get_reference(revs, arg, sha1, 0);
+		it = get_reference(revs, arg, &oid, 0);
 		if (!it && revs->ignore_missing)
 			return 0;
 		if (it->type != OBJ_TAG)
 			break;
 		if (!((struct tag*)it)->tagged)
 			return 0;
-		hashcpy(sha1, ((struct tag*)it)->tagged->oid.hash);
+		oidcpy(&oid, &((struct tag*)it)->tagged->oid);
 	}
 	if (it->type != OBJ_COMMIT)
 		return 0;
@@ -1389,16 +1389,16 @@ static void prepare_show_merge(struct rev_info *revs)
 {
 	struct commit_list *bases;
 	struct commit *head, *other;
-	unsigned char sha1[20];
+	struct object_id oid;
 	const char **prune = NULL;
 	int i, prune_num = 1; /* counting terminating NULL */
 
-	if (get_sha1("HEAD", sha1))
+	if (get_oid("HEAD", &oid))
 		die("--merge without HEAD?");
-	head = lookup_commit_or_die(sha1, "HEAD");
-	if (get_sha1("MERGE_HEAD", sha1))
+	head = lookup_commit_or_die(&oid, "HEAD");
+	if (get_oid("MERGE_HEAD", &oid))
 		die("--merge without MERGE_HEAD?");
-	other = lookup_commit_or_die(sha1, "MERGE_HEAD");
+	other = lookup_commit_or_die(&oid, "MERGE_HEAD");
 	add_pending_object(revs, &head->object, "HEAD");
 	add_pending_object(revs, &other->object, "MERGE_HEAD");
 	bases = get_merge_bases(head, other);
@@ -1434,7 +1434,7 @@ int handle_revision_arg(const char *arg_, struct rev_info *revs, int flags, unsi
 	struct object_context oc;
 	char *dotdot;
 	struct object *object;
-	unsigned char sha1[20];
+	struct object_id oid;
 	int local_flags;
 	const char *arg = arg_;
 	int cant_be_filename = revarg_opt & REVARG_CANNOT_BE_FILENAME;
@@ -1444,7 +1444,7 @@ int handle_revision_arg(const char *arg_, struct rev_info *revs, int flags, unsi
 
 	dotdot = strstr(arg, "..");
 	if (dotdot) {
-		unsigned char from_sha1[20];
+		struct object_id from_oid;
 		const char *next = dotdot + 2;
 		const char *this = arg;
 		int symmetric = *next == '.';
@@ -1470,8 +1470,8 @@ int handle_revision_arg(const char *arg_, struct rev_info *revs, int flags, unsi
 				return -1;
 			}
 		}
-		if (!get_sha1_committish(this, from_sha1) &&
-		    !get_sha1_committish(next, sha1)) {
+		if (!get_sha1_committish(this, from_oid.hash) &&
+		    !get_sha1_committish(next, oid.hash)) {
 			struct object *a_obj, *b_obj;
 
 			if (!cant_be_filename) {
@@ -1479,8 +1479,8 @@ int handle_revision_arg(const char *arg_, struct rev_info *revs, int flags, unsi
 				verify_non_filename(revs->prefix, arg);
 			}
 
-			a_obj = parse_object(from_sha1);
-			b_obj = parse_object(sha1);
+			a_obj = parse_object(&from_oid);
+			b_obj = parse_object(&oid);
 			if (!a_obj || !b_obj) {
 			missing:
 				if (revs->ignore_missing)
@@ -1500,10 +1500,10 @@ int handle_revision_arg(const char *arg_, struct rev_info *revs, int flags, unsi
 
 				a = (a_obj->type == OBJ_COMMIT
 				     ? (struct commit *)a_obj
-				     : lookup_commit_reference(a_obj->oid.hash));
+				     : lookup_commit_reference(&a_obj->oid));
 				b = (b_obj->type == OBJ_COMMIT
 				     ? (struct commit *)b_obj
-				     : lookup_commit_reference(b_obj->oid.hash));
+				     : lookup_commit_reference(&b_obj->oid));
 				if (!a || !b)
 					goto missing;
 				exclude = get_merge_bases(a, b);
@@ -1568,11 +1568,11 @@ int handle_revision_arg(const char *arg_, struct rev_info *revs, int flags, unsi
 	if (revarg_opt & REVARG_COMMITTISH)
 		get_sha1_flags = GET_SHA1_COMMITTISH;
 
-	if (get_sha1_with_context(arg, get_sha1_flags, sha1, &oc))
+	if (get_sha1_with_context(arg, get_sha1_flags, oid.hash, &oc))
 		return revs->ignore_missing ? 0 : -1;
 	if (!cant_be_filename)
 		verify_non_filename(revs->prefix, arg);
-	object = get_reference(revs, arg, sha1, flags ^ local_flags);
+	object = get_reference(revs, arg, &oid, flags ^ local_flags);
 	add_rev_cmdline(revs, object, arg_, REV_CMD_REV, flags ^ local_flags);
 	add_pending_object_with_mode(revs, object, arg, oc.mode);
 	return 0;
@@ -2287,12 +2287,12 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 	if (revs->show_merge)
 		prepare_show_merge(revs);
 	if (revs->def && !revs->pending.nr && !got_rev_arg) {
-		unsigned char sha1[20];
+		struct object_id oid;
 		struct object *object;
 		struct object_context oc;
-		if (get_sha1_with_context(revs->def, 0, sha1, &oc))
+		if (get_sha1_with_context(revs->def, 0, oid.hash, &oc))
 			diagnose_missing_default(revs->def);
-		object = get_reference(revs, revs->def, sha1, 0);
+		object = get_reference(revs, revs->def, &oid, 0);
 		add_pending_object_with_mode(revs, object, revs->def, oc.mode);
 	}
 
diff --git a/revision.h b/revision.h
index 0d9e68b36e..a91dd3d5d9 100644
--- a/revision.h
+++ b/revision.h
@@ -263,9 +263,9 @@ extern void show_object_with_name(FILE *, struct object *, const char *);
 
 extern void add_pending_object(struct rev_info *revs,
 			       struct object *obj, const char *name);
-extern void add_pending_sha1(struct rev_info *revs,
-			     const char *name, const unsigned char *sha1,
-			     unsigned int flags);
+extern void add_pending_oid(struct rev_info *revs,
+			    const char *name, const struct object_id *oid,
+			    unsigned int flags);
 
 extern void add_head_to_pending(struct rev_info *);
 extern void add_reflogs_to_pending(struct rev_info *, unsigned int flags);
diff --git a/sequencer.c b/sequencer.c
index 0fa3fb14f7..9dfedbc243 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -344,7 +344,7 @@ static int read_oneliner(struct strbuf *buf,
 
 static struct tree *empty_tree(void)
 {
-	return lookup_tree(EMPTY_TREE_SHA1_BIN);
+	return lookup_tree(&empty_tree_oid);
 }
 
 static int error_dirty_index(struct replay_opts *opts)
@@ -374,7 +374,7 @@ static void update_abort_safety_file(void)
 		write_file(git_path_abort_safety_file(), "%s", "");
 }
 
-static int fast_forward_to(const unsigned char *to, const unsigned char *from,
+static int fast_forward_to(const struct object_id *to, const struct object_id *from,
 			int unborn, struct replay_opts *opts)
 {
 	struct ref_transaction *transaction;
@@ -390,7 +390,7 @@ static int fast_forward_to(const unsigned char *to, const unsigned char *from,
 	transaction = ref_transaction_begin(&err);
 	if (!transaction ||
 	    ref_transaction_update(transaction, "HEAD",
-				   to, unborn ? null_sha1 : from,
+				   to->hash, unborn ? null_sha1 : from->hash,
 				   0, sb.buf, &err) ||
 	    ref_transaction_commit(transaction, &err)) {
 		ref_transaction_free(transaction);
@@ -426,7 +426,7 @@ void append_conflicts_hint(struct strbuf *msgbuf)
 
 static int do_recursive_merge(struct commit *base, struct commit *next,
 			      const char *base_label, const char *next_label,
-			      unsigned char *head, struct strbuf *msgbuf,
+			      struct object_id *head, struct strbuf *msgbuf,
 			      struct replay_opts *opts)
 {
 	struct merge_options o;
@@ -482,13 +482,13 @@ static int do_recursive_merge(struct commit *base, struct commit *next,
 
 static int is_index_unchanged(void)
 {
-	unsigned char head_sha1[20];
+	struct object_id head_oid;
 	struct commit *head_commit;
 
-	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, head_sha1, NULL))
+	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, head_oid.hash, NULL))
 		return error(_("could not resolve HEAD commit\n"));
 
-	head_commit = lookup_commit(head_sha1);
+	head_commit = lookup_commit(&head_oid);
 
 	/*
 	 * If head_commit is NULL, check_commit, called from
@@ -508,7 +508,8 @@ static int is_index_unchanged(void)
 		if (cache_tree_update(&the_index, 0))
 			return error(_("unable to update cache tree\n"));
 
-	return !hashcmp(active_cache_tree->sha1, head_commit->tree->object.oid.hash);
+	return !oidcmp(&active_cache_tree->oid,
+		       &head_commit->tree->object.oid);
 }
 
 static int write_author_script(const char *message)
@@ -834,13 +835,13 @@ static int update_squash_messages(enum todo_command command,
 		strbuf_splice(&buf, 0, eol - buf.buf, header.buf, header.len);
 		strbuf_release(&header);
 	} else {
-		unsigned char head[20];
+		struct object_id head;
 		struct commit *head_commit;
 		const char *head_message, *body;
 
-		if (get_sha1("HEAD", head))
+		if (get_oid("HEAD", &head))
 			return error(_("need a HEAD to fixup"));
-		if (!(head_commit = lookup_commit_reference(head)))
+		if (!(head_commit = lookup_commit_reference(&head)))
 			return error(_("could not read HEAD"));
 		if (!(head_message = get_commit_buffer(head_commit, NULL)))
 			return error(_("could not read HEAD's commit message"));
@@ -934,7 +935,7 @@ static int do_pick_commit(enum todo_command command, struct commit *commit,
 {
 	unsigned int flags = opts->edit ? EDIT_MSG : 0;
 	const char *msg_file = opts->edit ? NULL : git_path_merge_msg();
-	unsigned char head[20];
+	struct object_id head;
 	struct commit *base, *next, *parent;
 	const char *base_label, *next_label;
 	struct commit_message msg = { NULL, NULL, NULL, NULL };
@@ -948,12 +949,12 @@ static int do_pick_commit(enum todo_command command, struct commit *commit,
 		 * that represents the "current" state for merge-recursive
 		 * to work on.
 		 */
-		if (write_cache_as_tree(head, 0, NULL))
+		if (write_cache_as_tree(head.hash, 0, NULL))
 			return error(_("your index file is unmerged."));
 	} else {
-		unborn = get_sha1("HEAD", head);
+		unborn = get_oid("HEAD", &head);
 		if (unborn)
-			hashcpy(head, EMPTY_TREE_SHA1_BIN);
+			oidcpy(&head, &empty_tree_oid);
 		if (index_differs_from(unborn ? EMPTY_TREE_SHA1_HEX : "HEAD", 0, 0))
 			return error_dirty_index(opts);
 	}
@@ -989,11 +990,11 @@ static int do_pick_commit(enum todo_command command, struct commit *commit,
 			oid_to_hex(&commit->object.oid));
 
 	if (opts->allow_ff && !is_fixup(command) &&
-	    ((parent && !hashcmp(parent->object.oid.hash, head)) ||
+	    ((parent && !oidcmp(&parent->object.oid, &head)) ||
 	     (!parent && unborn))) {
 		if (is_rebase_i(opts))
 			write_author_script(msg.message);
-		res = fast_forward_to(commit->object.oid.hash, head, unborn,
+		res = fast_forward_to(&commit->object.oid, &head, unborn,
 			opts);
 		if (res || command != TODO_REWORD)
 			goto leave;
@@ -1081,7 +1082,7 @@ static int do_pick_commit(enum todo_command command, struct commit *commit,
 		res = -1;
 	else if (!opts->strategy || !strcmp(opts->strategy, "recursive") || command == TODO_REVERT) {
 		res = do_recursive_merge(base, next, base_label, next_label,
-					 head, &msgbuf, opts);
+					 &head, &msgbuf, opts);
 		if (res < 0)
 			return res;
 		res |= write_message(msgbuf.buf, msgbuf.len,
@@ -1097,7 +1098,7 @@ static int do_pick_commit(enum todo_command command, struct commit *commit,
 		commit_list_insert(next, &remotes);
 		res |= try_merge_command(opts->strategy,
 					 opts->xopts_nr, (const char **)opts->xopts,
-					common, sha1_to_hex(head), remotes);
+					common, oid_to_hex(&head), remotes);
 		free_commit_list(common);
 		free_commit_list(remotes);
 	}
@@ -1222,7 +1223,7 @@ static struct todo_item *append_new_todo(struct todo_list *todo_list)
 
 static int parse_insn_line(struct todo_item *item, const char *bol, char *eol)
 {
-	unsigned char commit_sha1[20];
+	struct object_id commit_oid;
 	char *end_of_object_name;
 	int i, saved, status, padding;
 
@@ -1271,7 +1272,7 @@ static int parse_insn_line(struct todo_item *item, const char *bol, char *eol)
 	end_of_object_name = (char *) bol + strcspn(bol, " \t\n");
 	saved = *end_of_object_name;
 	*end_of_object_name = '\0';
-	status = get_sha1(bol, commit_sha1);
+	status = get_oid(bol, &commit_oid);
 	*end_of_object_name = saved;
 
 	item->arg = end_of_object_name + strspn(end_of_object_name, " \t");
@@ -1280,7 +1281,7 @@ static int parse_insn_line(struct todo_item *item, const char *bol, char *eol)
 	if (status < 0)
 		return -1;
 
-	item->commit = lookup_commit_reference(commit_sha1);
+	item->commit = lookup_commit_reference(&commit_oid);
 	return !item->commit;
 }
 
@@ -2281,7 +2282,7 @@ static int single_pick(struct commit *cmit, struct replay_opts *opts)
 int sequencer_pick_revisions(struct replay_opts *opts)
 {
 	struct todo_list todo_list = TODO_LIST_INIT;
-	unsigned char sha1[20];
+	struct object_id oid;
 	int i, res;
 
 	assert(opts->revs);
@@ -2289,16 +2290,16 @@ int sequencer_pick_revisions(struct replay_opts *opts)
 		return -1;
 
 	for (i = 0; i < opts->revs->pending.nr; i++) {
-		unsigned char sha1[20];
+		struct object_id oid;
 		const char *name = opts->revs->pending.objects[i].name;
 
 		/* This happens when using --stdin. */
 		if (!strlen(name))
 			continue;
 
-		if (!get_sha1(name, sha1)) {
-			if (!lookup_commit_reference_gently(sha1, 1)) {
-				enum object_type type = sha1_object_info(sha1, NULL);
+		if (!get_oid(name, &oid)) {
+			if (!lookup_commit_reference_gently(&oid, 1)) {
+				enum object_type type = sha1_object_info(oid.hash, NULL);
 				return error(_("%s: can't cherry-pick a %s"),
 					name, typename(type));
 			}
@@ -2335,9 +2336,9 @@ int sequencer_pick_revisions(struct replay_opts *opts)
 	if (walk_revs_populate_todo(&todo_list, opts) ||
 			create_seq_dir() < 0)
 		return -1;
-	if (get_sha1("HEAD", sha1) && (opts->action == REPLAY_REVERT))
+	if (get_oid("HEAD", &oid) && (opts->action == REPLAY_REVERT))
 		return error(_("can't revert as initial commit"));
-	if (save_head(sha1_to_hex(sha1)))
+	if (save_head(oid_to_hex(&oid)))
 		return -1;
 	if (save_opts(opts))
 		return -1;
diff --git a/server-info.c b/server-info.c
index f6c1a3dfb0..6f865b73a3 100644
--- a/server-info.c
+++ b/server-info.c
@@ -53,7 +53,7 @@ static int add_info_ref(const char *path, const struct object_id *oid,
 			int flag, void *cb_data)
 {
 	FILE *fp = cb_data;
-	struct object *o = parse_object(oid->hash);
+	struct object *o = parse_object(oid);
 	if (!o)
 		return -1;
 
diff --git a/sha1_name.c b/sha1_name.c
index 35c1e2a9e3..389276e9d3 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -241,7 +241,7 @@ static int disambiguate_committish_only(const struct object_id *oid, void *cb_da
 		return 0;
 
 	/* We need to do this the hard way... */
-	obj = deref_tag(parse_object(oid->hash), NULL, 0);
+	obj = deref_tag(parse_object(oid), NULL, 0);
 	if (obj && obj->type == OBJ_COMMIT)
 		return 1;
 	return 0;
@@ -265,7 +265,7 @@ static int disambiguate_treeish_only(const struct object_id *oid, void *cb_data_
 		return 0;
 
 	/* We need to do this the hard way... */
-	obj = deref_tag(parse_object(oid->hash), NULL, 0);
+	obj = deref_tag(parse_object(oid), NULL, 0);
 	if (obj && (obj->type == OBJ_TREE || obj->type == OBJ_COMMIT))
 		return 1;
 	return 0;
@@ -354,14 +354,14 @@ static int show_ambiguous_object(const struct object_id *oid, void *data)
 
 	type = sha1_object_info(oid->hash, NULL);
 	if (type == OBJ_COMMIT) {
-		struct commit *commit = lookup_commit(oid->hash);
+		struct commit *commit = lookup_commit(oid);
 		if (commit) {
 			struct pretty_print_context pp = {0};
 			pp.date_mode.type = DATE_SHORT;
 			format_commit_message(commit, " %ad - %s", &desc, &pp);
 		}
 	} else if (type == OBJ_TAG) {
-		struct tag *tag = lookup_tag(oid->hash);
+		struct tag *tag = lookup_tag(oid);
 		if (!parse_tag(tag) && tag->tag)
 			strbuf_addf(&desc, " %s", tag->tag);
 	}
@@ -722,14 +722,14 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1,
 static int get_parent(const char *name, int len,
 		      unsigned char *result, int idx)
 {
-	unsigned char sha1[20];
-	int ret = get_sha1_1(name, len, sha1, GET_SHA1_COMMITTISH);
+	struct object_id oid;
+	int ret = get_sha1_1(name, len, oid.hash, GET_SHA1_COMMITTISH);
 	struct commit *commit;
 	struct commit_list *p;
 
 	if (ret)
 		return ret;
-	commit = lookup_commit_reference(sha1);
+	commit = lookup_commit_reference(&oid);
 	if (parse_commit(commit))
 		return -1;
 	if (!idx) {
@@ -750,14 +750,14 @@ static int get_parent(const char *name, int len,
 static int get_nth_ancestor(const char *name, int len,
 			    unsigned char *result, int generation)
 {
-	unsigned char sha1[20];
+	struct object_id oid;
 	struct commit *commit;
 	int ret;
 
-	ret = get_sha1_1(name, len, sha1, GET_SHA1_COMMITTISH);
+	ret = get_sha1_1(name, len, oid.hash, GET_SHA1_COMMITTISH);
 	if (ret)
 		return ret;
-	commit = lookup_commit_reference(sha1);
+	commit = lookup_commit_reference(&oid);
 	if (!commit)
 		return -1;
 
@@ -776,7 +776,7 @@ struct object *peel_to_type(const char *name, int namelen,
 	if (name && !namelen)
 		namelen = strlen(name);
 	while (1) {
-		if (!o || (!o->parsed && !parse_object(o->oid.hash)))
+		if (!o || (!o->parsed && !parse_object(&o->oid)))
 			return NULL;
 		if (expected_type == OBJ_ANY || o->type == expected_type)
 			return o;
@@ -798,7 +798,7 @@ struct object *peel_to_type(const char *name, int namelen,
 static int peel_onion(const char *name, int len, unsigned char *sha1,
 		      unsigned lookup_flags)
 {
-	unsigned char outer[20];
+	struct object_id outer;
 	const char *sp;
 	unsigned int expected_type = 0;
 	struct object *o;
@@ -846,15 +846,15 @@ static int peel_onion(const char *name, int len, unsigned char *sha1,
 	else if (expected_type == OBJ_TREE)
 		lookup_flags |= GET_SHA1_TREEISH;
 
-	if (get_sha1_1(name, sp - name - 2, outer, lookup_flags))
+	if (get_sha1_1(name, sp - name - 2, outer.hash, lookup_flags))
 		return -1;
 
-	o = parse_object(outer);
+	o = parse_object(&outer);
 	if (!o)
 		return -1;
 	if (!expected_type) {
 		o = deref_tag(o, name, sp - name - 2);
-		if (!o || (!o->parsed && !parse_object(o->oid.hash)))
+		if (!o || (!o->parsed && !parse_object(&o->oid)))
 			return -1;
 		hashcpy(sha1, o->oid.hash);
 		return 0;
@@ -981,7 +981,7 @@ static int handle_one_ref(const char *path, const struct object_id *oid,
 			  int flag, void *cb_data)
 {
 	struct commit_list **list = cb_data;
-	struct object *object = parse_object(oid->hash);
+	struct object *object = parse_object(oid);
 	if (!object)
 		return 0;
 	if (object->type == OBJ_TAG) {
@@ -1027,7 +1027,7 @@ static int get_sha1_oneline(const char *prefix, unsigned char *sha1,
 		int matches;
 
 		commit = pop_most_recent_commit(&list, ONELINE_SEEN);
-		if (!parse_object(commit->object.oid.hash))
+		if (!parse_object(&commit->object.oid))
 			continue;
 		buf = get_commit_buffer(commit, NULL);
 		p = strstr(buf, "\n\n");
@@ -1136,13 +1136,13 @@ int get_oid_mb(const char *name, struct object_id *oid)
 	}
 	if (st)
 		return st;
-	one = lookup_commit_reference_gently(oid_tmp.hash, 0);
+	one = lookup_commit_reference_gently(&oid_tmp, 0);
 	if (!one)
 		return -1;
 
 	if (get_sha1_committish(dots[3] ? (dots + 3) : "HEAD", oid_tmp.hash))
 		return -1;
-	two = lookup_commit_reference_gently(oid_tmp.hash, 0);
+	two = lookup_commit_reference_gently(&oid_tmp, 0);
 	if (!two)
 		return -1;
 	mbs = get_merge_bases(one, two);
diff --git a/shallow.c b/shallow.c
index 25b6db989b..6950cc24f0 100644
--- a/shallow.c
+++ b/shallow.c
@@ -27,13 +27,13 @@ void set_alternate_shallow_file(const char *path, int override)
 	alternate_shallow_file = xstrdup_or_null(path);
 }
 
-int register_shallow(const unsigned char *sha1)
+int register_shallow(const struct object_id *oid)
 {
 	struct commit_graft *graft =
 		xmalloc(sizeof(struct commit_graft));
-	struct commit *commit = lookup_commit(sha1);
+	struct commit *commit = lookup_commit(oid);
 
-	hashcpy(graft->oid.hash, sha1);
+	oidcpy(&graft->oid, oid);
 	graft->nr_parent = -1;
 	if (commit && commit->object.parsed)
 		commit->parents = NULL;
@@ -65,10 +65,10 @@ int is_repository_shallow(void)
 	is_shallow = 1;
 
 	while (fgets(buf, sizeof(buf), fp)) {
-		unsigned char sha1[20];
-		if (get_sha1_hex(buf, sha1))
+		struct object_id oid;
+		if (get_oid_hex(buf, &oid))
 			die("bad shallow line: %s", buf);
-		register_shallow(sha1);
+		register_shallow(&oid);
 	}
 	fclose(fp);
 	return is_shallow;
@@ -241,7 +241,7 @@ static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
 	if (graft->nr_parent != -1)
 		return 0;
 	if (data->flags & SEEN_ONLY) {
-		struct commit *c = lookup_commit(graft->oid.hash);
+		struct commit *c = lookup_commit(&graft->oid);
 		if (!c || !(c->object.flags & SEEN)) {
 			if (data->flags & VERBOSE)
 				printf("Removing %s from .git/shallow\n",
@@ -466,7 +466,7 @@ static uint32_t *paint_alloc(struct paint_info *info)
  * UNINTERESTING or BOTTOM is hit. Set the id-th bit in ref_bitmap for
  * all walked commits.
  */
-static void paint_down(struct paint_info *info, const unsigned char *sha1,
+static void paint_down(struct paint_info *info, const struct object_id *oid,
 		       unsigned int id)
 {
 	unsigned int i, nr;
@@ -475,7 +475,7 @@ static void paint_down(struct paint_info *info, const unsigned char *sha1,
 	size_t bitmap_size = st_mult(sizeof(uint32_t), bitmap_nr);
 	uint32_t *tmp = xmalloc(bitmap_size); /* to be freed before return */
 	uint32_t *bitmap = paint_alloc(info);
-	struct commit *c = lookup_commit_reference_gently(sha1, 1);
+	struct commit *c = lookup_commit_reference_gently(oid, 1);
 	if (!c)
 		return;
 	memset(bitmap, 0, bitmap_size);
@@ -531,7 +531,7 @@ static void paint_down(struct paint_info *info, const unsigned char *sha1,
 static int mark_uninteresting(const char *refname, const struct object_id *oid,
 			      int flags, void *cb_data)
 {
-	struct commit *commit = lookup_commit_reference_gently(oid->hash, 1);
+	struct commit *commit = lookup_commit_reference_gently(oid, 1);
 	if (!commit)
 		return 0;
 	commit->object.flags |= UNINTERESTING;
@@ -599,18 +599,18 @@ void assign_shallow_commits_to_refs(struct shallow_info *info,
 
 	/* Mark potential bottoms so we won't go out of bound */
 	for (i = 0; i < nr_shallow; i++) {
-		struct commit *c = lookup_commit(oid[shallow[i]].hash);
+		struct commit *c = lookup_commit(&oid[shallow[i]]);
 		c->object.flags |= BOTTOM;
 	}
 
 	for (i = 0; i < ref->nr; i++)
-		paint_down(&pi, ref->oid[i].hash, i);
+		paint_down(&pi, ref->oid + i, i);
 
 	if (used) {
 		int bitmap_size = ((pi.nr_bits + 31) / 32) * sizeof(uint32_t);
 		memset(used, 0, sizeof(*used) * info->shallow->nr);
 		for (i = 0; i < nr_shallow; i++) {
-			const struct commit *c = lookup_commit(oid[shallow[i]].hash);
+			const struct commit *c = lookup_commit(&oid[shallow[i]]);
 			uint32_t **map = ref_bitmap_at(&pi.ref_bitmap, c);
 			if (*map)
 				used[shallow[i]] = xmemdupz(*map, bitmap_size);
@@ -641,7 +641,7 @@ static int add_ref(const char *refname, const struct object_id *oid,
 {
 	struct commit_array *ca = cb_data;
 	ALLOC_GROW(ca->commits, ca->nr + 1, ca->alloc);
-	ca->commits[ca->nr] = lookup_commit_reference_gently(oid->hash, 1);
+	ca->commits[ca->nr] = lookup_commit_reference_gently(oid, 1);
 	if (ca->commits[ca->nr])
 		ca->nr++;
 	return 0;
@@ -679,7 +679,7 @@ static void post_assign_shallow(struct shallow_info *info,
 	for (i = dst = 0; i < info->nr_theirs; i++) {
 		if (i != dst)
 			info->theirs[dst] = info->theirs[i];
-		c = lookup_commit(oid[info->theirs[i]].hash);
+		c = lookup_commit(&oid[info->theirs[i]]);
 		bitmap = ref_bitmap_at(ref_bitmap, c);
 		if (!*bitmap)
 			continue;
@@ -700,7 +700,7 @@ static void post_assign_shallow(struct shallow_info *info,
 	for (i = dst = 0; i < info->nr_ours; i++) {
 		if (i != dst)
 			info->ours[dst] = info->ours[i];
-		c = lookup_commit(oid[info->ours[i]].hash);
+		c = lookup_commit(&oid[info->ours[i]]);
 		bitmap = ref_bitmap_at(ref_bitmap, c);
 		if (!*bitmap)
 			continue;
@@ -722,7 +722,7 @@ static void post_assign_shallow(struct shallow_info *info,
 int delayed_reachability_test(struct shallow_info *si, int c)
 {
 	if (si->need_reachability_test[c]) {
-		struct commit *commit = lookup_commit(si->shallow->oid[c].hash);
+		struct commit *commit = lookup_commit(&si->shallow->oid[c]);
 
 		if (!si->commits) {
 			struct commit_array ca;
diff --git a/submodule.c b/submodule.c
index 5bb7e7a033..9d5ec5f084 100644
--- a/submodule.c
+++ b/submodule.c
@@ -447,8 +447,8 @@ static void show_submodule_header(FILE *f, const char *path,
 	 * Attempt to lookup the commit references, and determine if this is
 	 * a fast forward or fast backwards update.
 	 */
-	*left = lookup_commit_reference(one->hash);
-	*right = lookup_commit_reference(two->hash);
+	*left = lookup_commit_reference(one);
+	*right = lookup_commit_reference(two);
 
 	/*
 	 * Warn about missing commits in the submodule project, but only if
@@ -723,7 +723,7 @@ static int check_has_commit(const struct object_id *oid, void *data)
 {
 	int *has_commit = data;
 
-	if (!lookup_commit_reference(oid->hash))
+	if (!lookup_commit_reference(oid))
 		*has_commit = 0;
 
 	return 0;
@@ -1584,9 +1584,9 @@ static void print_commit(struct commit *commit)
 #define MERGE_WARNING(path, msg) \
 	warning("Failed to merge submodule %s (%s)", path, msg);
 
-int merge_submodule(unsigned char result[20], const char *path,
-		    const unsigned char base[20], const unsigned char a[20],
-		    const unsigned char b[20], int search)
+int merge_submodule(struct object_id *result, const char *path,
+		    const struct object_id *base, const struct object_id *a,
+		    const struct object_id *b, int search)
 {
 	struct commit *commit_base, *commit_a, *commit_b;
 	int parent_count;
@@ -1595,14 +1595,14 @@ int merge_submodule(unsigned char result[20], const char *path,
 	int i;
 
 	/* store a in result in case we fail */
-	hashcpy(result, a);
+	oidcpy(result, a);
 
 	/* we can not handle deletion conflicts */
-	if (is_null_sha1(base))
+	if (is_null_oid(base))
 		return 0;
-	if (is_null_sha1(a))
+	if (is_null_oid(a))
 		return 0;
-	if (is_null_sha1(b))
+	if (is_null_oid(b))
 		return 0;
 
 	if (add_submodule_odb(path)) {
@@ -1626,11 +1626,11 @@ int merge_submodule(unsigned char result[20], const char *path,
 
 	/* Case #1: a is contained in b or vice versa */
 	if (in_merge_bases(commit_a, commit_b)) {
-		hashcpy(result, b);
+		oidcpy(result, b);
 		return 1;
 	}
 	if (in_merge_bases(commit_b, commit_a)) {
-		hashcpy(result, a);
+		oidcpy(result, a);
 		return 1;
 	}
 
diff --git a/submodule.h b/submodule.h
index 1277480add..89c2ed2191 100644
--- a/submodule.h
+++ b/submodule.h
@@ -84,10 +84,10 @@ extern int submodule_uses_gitfile(const char *path);
 #define SUBMODULE_REMOVAL_IGNORE_UNTRACKED (1<<1)
 #define SUBMODULE_REMOVAL_IGNORE_IGNORED_UNTRACKED (1<<2)
 extern int bad_to_remove_submodule(const char *path, unsigned flags);
-extern int merge_submodule(unsigned char result[20], const char *path,
-			   const unsigned char base[20],
-			   const unsigned char a[20],
-			   const unsigned char b[20], int search);
+extern int merge_submodule(struct object_id *result, const char *path,
+			   const struct object_id *base,
+			   const struct object_id *a,
+			   const struct object_id *b, int search);
 extern int find_unpushed_submodules(struct oid_array *commits,
 				    const char *remotes_name,
 				    struct string_list *needs_pushing);
diff --git a/t/helper/test-dump-cache-tree.c b/t/helper/test-dump-cache-tree.c
index 7af116d49e..ebf3aab22d 100644
--- a/t/helper/test-dump-cache-tree.c
+++ b/t/helper/test-dump-cache-tree.c
@@ -10,7 +10,7 @@ static void dump_one(struct cache_tree *it, const char *pfx, const char *x)
 		       "invalid", x, pfx, it->subtree_nr);
 	else
 		printf("%s %s%s (%d entries, %d subtrees)\n",
-		       sha1_to_hex(it->sha1), x, pfx,
+		       oid_to_hex(&it->oid), x, pfx,
 		       it->entry_count, it->subtree_nr);
 }
 
@@ -32,7 +32,7 @@ static int dump_cache_tree(struct cache_tree *it,
 	}
 	else {
 		dump_one(it, pfx, "");
-		if (hashcmp(it->sha1, ref->sha1) ||
+		if (oidcmp(&it->oid, &ref->oid) ||
 		    ref->entry_count != it->entry_count ||
 		    ref->subtree_nr != it->subtree_nr) {
 			/* claims to be valid but is lying */
diff --git a/t/helper/test-match-trees.c b/t/helper/test-match-trees.c
index e939502863..356d8edef1 100644
--- a/t/helper/test-match-trees.c
+++ b/t/helper/test-match-trees.c
@@ -12,10 +12,10 @@ int cmd_main(int ac, const char **av)
 		die("cannot parse %s as an object name", av[1]);
 	if (get_oid(av[2], &hash2))
 		die("cannot parse %s as an object name", av[2]);
-	one = parse_tree_indirect(hash1.hash);
+	one = parse_tree_indirect(&hash1);
 	if (!one)
 		die("not a tree-ish %s", av[1]);
-	two = parse_tree_indirect(hash2.hash);
+	two = parse_tree_indirect(&hash2);
 	if (!two)
 		die("not a tree-ish %s", av[2]);
 
diff --git a/tag.c b/tag.c
index d71b67e8d8..47f60ae151 100644
--- a/tag.c
+++ b/tag.c
@@ -66,7 +66,7 @@ struct object *deref_tag(struct object *o, const char *warn, int warnlen)
 {
 	while (o && o->type == OBJ_TAG)
 		if (((struct tag *)o)->tagged)
-			o = parse_object(((struct tag *)o)->tagged->oid.hash);
+			o = parse_object(&((struct tag *)o)->tagged->oid);
 		else
 			o = NULL;
 	if (!o && warn) {
@@ -80,7 +80,7 @@ struct object *deref_tag(struct object *o, const char *warn, int warnlen)
 struct object *deref_tag_noverify(struct object *o)
 {
 	while (o && o->type == OBJ_TAG) {
-		o = parse_object(o->oid.hash);
+		o = parse_object(&o->oid);
 		if (o && o->type == OBJ_TAG && ((struct tag *)o)->tagged)
 			o = ((struct tag *)o)->tagged;
 		else
@@ -89,11 +89,11 @@ struct object *deref_tag_noverify(struct object *o)
 	return o;
 }
 
-struct tag *lookup_tag(const unsigned char *sha1)
+struct tag *lookup_tag(const struct object_id *oid)
 {
-	struct object *obj = lookup_object(sha1);
+	struct object *obj = lookup_object(oid->hash);
 	if (!obj)
-		return create_object(sha1, alloc_tag_node());
+		return create_object(oid->hash, alloc_tag_node());
 	return object_as_type(obj, OBJ_TAG, 0);
 }
 
@@ -116,7 +116,7 @@ static timestamp_t parse_tag_date(const char *buf, const char *tail)
 
 int parse_tag_buffer(struct tag *item, const void *data, unsigned long size)
 {
-	unsigned char sha1[20];
+	struct object_id oid;
 	char type[20];
 	const char *bufptr = data;
 	const char *tail = bufptr + size;
@@ -126,11 +126,10 @@ int parse_tag_buffer(struct tag *item, const void *data, unsigned long size)
 		return 0;
 	item->object.parsed = 1;
 
-	if (size < 64)
+	if (size < GIT_SHA1_HEXSZ + 24)
 		return -1;
-	if (memcmp("object ", bufptr, 7) || get_sha1_hex(bufptr + 7, sha1) || bufptr[47] != '\n')
+	if (memcmp("object ", bufptr, 7) || parse_oid_hex(bufptr + 7, &oid, &bufptr) || *bufptr++ != '\n')
 		return -1;
-	bufptr += 48; /* "object " + sha1 + "\n" */
 
 	if (!starts_with(bufptr, "type "))
 		return -1;
@@ -143,13 +142,13 @@ int parse_tag_buffer(struct tag *item, const void *data, unsigned long size)
 	bufptr = nl + 1;
 
 	if (!strcmp(type, blob_type)) {
-		item->tagged = &lookup_blob(sha1)->object;
+		item->tagged = &lookup_blob(&oid)->object;
 	} else if (!strcmp(type, tree_type)) {
-		item->tagged = &lookup_tree(sha1)->object;
+		item->tagged = &lookup_tree(&oid)->object;
 	} else if (!strcmp(type, commit_type)) {
-		item->tagged = &lookup_commit(sha1)->object;
+		item->tagged = &lookup_commit(&oid)->object;
 	} else if (!strcmp(type, tag_type)) {
-		item->tagged = &lookup_tag(sha1)->object;
+		item->tagged = &lookup_tag(&oid)->object;
 	} else {
 		error("Unknown type %s", type);
 		item->tagged = NULL;
diff --git a/tag.h b/tag.h
index 2abb3726fb..fdfcb4a84a 100644
--- a/tag.h
+++ b/tag.h
@@ -12,7 +12,7 @@ struct tag {
 	timestamp_t date;
 };
 
-extern struct tag *lookup_tag(const unsigned char *sha1);
+extern struct tag *lookup_tag(const struct object_id *oid);
 extern int parse_tag_buffer(struct tag *item, const void *data, unsigned long size);
 extern int parse_tag(struct tag *item);
 extern struct object *deref_tag(struct object *, const char *, int);
diff --git a/transport.c b/transport.c
index 4d33138a75..9bfcf870f9 100644
--- a/transport.c
+++ b/transport.c
@@ -87,7 +87,7 @@ static struct ref *get_refs_from_bundle(struct transport *transport, int for_pus
 	for (i = 0; i < data->header.references.nr; i++) {
 		struct ref_list_entry *e = data->header.references.list + i;
 		struct ref *ref = alloc_ref(e->name);
-		hashcpy(ref->old_oid.hash, e->sha1);
+		oidcpy(&ref->old_oid, &e->oid);
 		ref->next = result;
 		result = ref;
 	}
diff --git a/tree.c b/tree.c
index ce345c5511..603b29ee80 100644
--- a/tree.c
+++ b/tree.c
@@ -58,7 +58,7 @@ static int read_tree_1(struct tree *tree, struct strbuf *base,
 {
 	struct tree_desc desc;
 	struct name_entry entry;
-	unsigned char sha1[20];
+	struct object_id oid;
 	int len, oldlen = base->len;
 	enum interesting retval = entry_not_interesting;
 
@@ -87,11 +87,11 @@ static int read_tree_1(struct tree *tree, struct strbuf *base,
 		}
 
 		if (S_ISDIR(entry.mode))
-			hashcpy(sha1, entry.oid->hash);
+			oidcpy(&oid, entry.oid);
 		else if (S_ISGITLINK(entry.mode)) {
 			struct commit *commit;
 
-			commit = lookup_commit(entry.oid->hash);
+			commit = lookup_commit(entry.oid);
 			if (!commit)
 				die("Commit %s in submodule path %s%s not found",
 				    oid_to_hex(entry.oid),
@@ -102,7 +102,7 @@ static int read_tree_1(struct tree *tree, struct strbuf *base,
 				    oid_to_hex(entry.oid),
 				    base->buf, entry.path);
 
-			hashcpy(sha1, commit->tree->object.oid.hash);
+			oidcpy(&oid, &commit->tree->object.oid);
 		}
 		else
 			continue;
@@ -110,7 +110,7 @@ static int read_tree_1(struct tree *tree, struct strbuf *base,
 		len = tree_entry_len(&entry);
 		strbuf_add(base, entry.path, len);
 		strbuf_addch(base, '/');
-		retval = read_tree_1(lookup_tree(sha1),
+		retval = read_tree_1(lookup_tree(&oid),
 				     base, stage, pathspec,
 				     fn, context);
 		strbuf_setlen(base, oldlen);
@@ -184,11 +184,11 @@ int read_tree(struct tree *tree, int stage, struct pathspec *match)
 	return 0;
 }
 
-struct tree *lookup_tree(const unsigned char *sha1)
+struct tree *lookup_tree(const struct object_id *oid)
 {
-	struct object *obj = lookup_object(sha1);
+	struct object *obj = lookup_object(oid->hash);
 	if (!obj)
-		return create_object(sha1, alloc_tree_node());
+		return create_object(oid->hash, alloc_tree_node());
 	return object_as_type(obj, OBJ_TREE, 0);
 }
 
@@ -232,9 +232,9 @@ void free_tree_buffer(struct tree *tree)
 	tree->object.parsed = 0;
 }
 
-struct tree *parse_tree_indirect(const unsigned char *sha1)
+struct tree *parse_tree_indirect(const struct object_id *oid)
 {
-	struct object *obj = parse_object(sha1);
+	struct object *obj = parse_object(oid);
 	do {
 		if (!obj)
 			return NULL;
@@ -247,6 +247,6 @@ struct tree *parse_tree_indirect(const unsigned char *sha1)
 		else
 			return NULL;
 		if (!obj->parsed)
-			parse_object(obj->oid.hash);
+			parse_object(&obj->oid);
 	} while (1);
 }
diff --git a/tree.h b/tree.h
index d24786cba2..0d4734b94b 100644
--- a/tree.h
+++ b/tree.h
@@ -12,7 +12,7 @@ struct tree {
 	unsigned long size;
 };
 
-struct tree *lookup_tree(const unsigned char *sha1);
+struct tree *lookup_tree(const struct object_id *oid);
 
 int parse_tree_buffer(struct tree *item, void *buffer, unsigned long size);
 
@@ -24,7 +24,7 @@ static inline int parse_tree(struct tree *tree)
 void free_tree_buffer(struct tree *tree);
 
 /* Parses and returns the tree in the given ent, chasing tags and commits. */
-struct tree *parse_tree_indirect(const unsigned char *sha1);
+struct tree *parse_tree_indirect(const struct object_id *oid);
 
 #define READ_TREE_RECURSIVE 1
 typedef int (*read_tree_fn_t)(const unsigned char *, struct strbuf *, const char *, unsigned int, int, void *);
diff --git a/upload-pack.c b/upload-pack.c
index 97da13e6a5..5330c02c14 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -286,19 +286,19 @@ static void create_pack_file(void)
 	die("git upload-pack: %s", abort_msg);
 }
 
-static int got_sha1(const char *hex, unsigned char *sha1)
+static int got_oid(const char *hex, struct object_id *oid)
 {
 	struct object *o;
 	int we_knew_they_have = 0;
 
-	if (get_sha1_hex(hex, sha1))
+	if (get_oid_hex(hex, oid))
 		die("git upload-pack: expected SHA1 object, got '%s'", hex);
-	if (!has_sha1_file(sha1))
+	if (!has_object_file(oid))
 		return -1;
 
-	o = parse_object(sha1);
+	o = parse_object(oid);
 	if (!o)
-		die("oops (%s)", sha1_to_hex(sha1));
+		die("oops (%s)", oid_to_hex(oid));
 	if (o->type == OBJ_COMMIT) {
 		struct commit_list *parents;
 		struct commit *commit = (struct commit *)o;
@@ -334,7 +334,7 @@ static int reachable(struct commit *want)
 			break;
 		}
 		if (!commit->object.parsed)
-			parse_object(commit->object.oid.hash);
+			parse_object(&commit->object.oid);
 		if (commit->object.flags & REACHABLE)
 			continue;
 		commit->object.flags |= REACHABLE;
@@ -382,8 +382,8 @@ static int ok_to_give_up(void)
 
 static int get_common_commits(void)
 {
-	unsigned char sha1[20];
-	char last_hex[41];
+	struct object_id oid;
+	char last_hex[GIT_MAX_HEXSZ + 1];
 	int got_common = 0;
 	int got_other = 0;
 	int sent_ready = 0;
@@ -416,11 +416,11 @@ static int get_common_commits(void)
 			continue;
 		}
 		if (skip_prefix(line, "have ", &arg)) {
-			switch (got_sha1(arg, sha1)) {
+			switch (got_oid(arg, &oid)) {
 			case -1: /* they have what we do not */
 				got_other = 1;
 				if (multi_ack && ok_to_give_up()) {
-					const char *hex = sha1_to_hex(sha1);
+					const char *hex = oid_to_hex(&oid);
 					if (multi_ack == 2) {
 						sent_ready = 1;
 						packet_write_fmt(1, "ACK %s ready\n", hex);
@@ -430,7 +430,7 @@ static int get_common_commits(void)
 				break;
 			default:
 				got_common = 1;
-				memcpy(last_hex, sha1_to_hex(sha1), 41);
+				memcpy(last_hex, oid_to_hex(&oid), 41);
 				if (multi_ack == 2)
 					packet_write_fmt(1, "ACK %s common\n", last_hex);
 				else if (multi_ack)
@@ -492,7 +492,7 @@ static int do_reachable_revlist(struct child_process *cmd,
 		goto error;
 
 	namebuf[0] = '^';
-	namebuf[41] = '\n';
+	namebuf[GIT_SHA1_HEXSZ + 1] = '\n';
 	for (i = get_max_object_index(); 0 < i; ) {
 		o = get_indexed_object(--i);
 		if (!o)
@@ -502,10 +502,10 @@ static int do_reachable_revlist(struct child_process *cmd,
 		if (!is_our_ref(o))
 			continue;
 		memcpy(namebuf + 1, oid_to_hex(&o->oid), GIT_SHA1_HEXSZ);
-		if (write_in_full(cmd->in, namebuf, 42) < 0)
+		if (write_in_full(cmd->in, namebuf, GIT_SHA1_HEXSZ + 2) < 0)
 			goto error;
 	}
-	namebuf[40] = '\n';
+	namebuf[GIT_SHA1_HEXSZ] = '\n';
 	for (i = 0; i < src->nr; i++) {
 		o = src->objects[i].item;
 		if (is_our_ref(o)) {
@@ -516,7 +516,7 @@ static int do_reachable_revlist(struct child_process *cmd,
 		if (reachable && o->type == OBJ_COMMIT)
 			o->flags |= TMP_MARK;
 		memcpy(namebuf, oid_to_hex(&o->oid), GIT_SHA1_HEXSZ);
-		if (write_in_full(cmd->in, namebuf, 41) < 0)
+		if (write_in_full(cmd->in, namebuf, GIT_SHA1_HEXSZ + 1) < 0)
 			goto error;
 	}
 	close(cmd->in);
@@ -642,7 +642,7 @@ static void send_shallow(struct commit_list *result)
 		if (!(object->flags & (CLIENT_SHALLOW|NOT_SHALLOW))) {
 			packet_write_fmt(1, "shallow %s",
 					 oid_to_hex(&object->oid));
-			register_shallow(object->oid.hash);
+			register_shallow(&object->oid);
 			shallow_nr++;
 		}
 		result = result->next;
@@ -667,7 +667,7 @@ static void send_unshallow(const struct object_array *shallows)
 			 * parse and add the parents to the want list, then
 			 * re-register it.
 			 */
-			unregister_shallow(object->oid.hash);
+			unregister_shallow(&object->oid);
 			object->parsed = 0;
 			parse_commit_or_die((struct commit *)object);
 			parents = ((struct commit *)object)->parents;
@@ -679,7 +679,7 @@ static void send_unshallow(const struct object_array *shallows)
 			add_object_array(object, NULL, &extra_edge_obj);
 		}
 		/* make sure commit traversal conforms to client */
-		register_shallow(object->oid.hash);
+		register_shallow(&object->oid);
 	}
 }
 
@@ -742,7 +742,7 @@ static void receive_needs(void)
 	for (;;) {
 		struct object *o;
 		const char *features;
-		unsigned char sha1_buf[20];
+		struct object_id oid_buf;
 		char *line = packet_read_line(0, NULL);
 		const char *arg;
 
@@ -751,15 +751,15 @@ static void receive_needs(void)
 			break;
 
 		if (skip_prefix(line, "shallow ", &arg)) {
-			unsigned char sha1[20];
+			struct object_id oid;
 			struct object *object;
-			if (get_sha1_hex(arg, sha1))
+			if (get_oid_hex(arg, &oid))
 				die("invalid shallow line: %s", line);
-			object = parse_object(sha1);
+			object = parse_object(&oid);
 			if (!object)
 				continue;
 			if (object->type != OBJ_COMMIT)
-				die("invalid shallow object %s", sha1_to_hex(sha1));
+				die("invalid shallow object %s", oid_to_hex(&oid));
 			if (!(object->flags & CLIENT_SHALLOW)) {
 				object->flags |= CLIENT_SHALLOW;
 				add_object_array(object, NULL, &shallows);
@@ -785,8 +785,8 @@ static void receive_needs(void)
 		}
 		if (skip_prefix(line, "deepen-not ", &arg)) {
 			char *ref = NULL;
-			unsigned char sha1[20];
-			if (expand_ref(arg, strlen(arg), sha1, &ref) != 1)
+			struct object_id oid;
+			if (expand_ref(arg, strlen(arg), oid.hash, &ref) != 1)
 				die("git upload-pack: ambiguous deepen-not: %s", line);
 			string_list_append(&deepen_not, ref);
 			free(ref);
@@ -794,7 +794,7 @@ static void receive_needs(void)
 			continue;
 		}
 		if (!skip_prefix(line, "want ", &arg) ||
-		    get_sha1_hex(arg, sha1_buf))
+		    get_oid_hex(arg, &oid_buf))
 			die("git upload-pack: protocol error, "
 			    "expected to get sha, not '%s'", line);
 
@@ -821,13 +821,13 @@ static void receive_needs(void)
 		if (parse_feature_request(features, "include-tag"))
 			use_include_tag = 1;
 
-		o = parse_object(sha1_buf);
+		o = parse_object(&oid_buf);
 		if (!o) {
 			packet_write_fmt(1,
 					 "ERR upload-pack: not our ref %s",
-					 sha1_to_hex(sha1_buf));
+					 oid_to_hex(&oid_buf));
 			die("git upload-pack: not our ref %s",
-			    sha1_to_hex(sha1_buf));
+			    oid_to_hex(&oid_buf));
 		}
 		if (!(o->flags & WANTED)) {
 			o->flags |= WANTED;
@@ -883,7 +883,7 @@ static void receive_needs(void)
 		if (shallows.nr > 0) {
 			int i;
 			for (i = 0; i < shallows.nr; i++)
-				register_shallow(shallows.objects[i].item->oid.hash);
+				register_shallow(&shallows.objects[i].item->oid);
 		}
 
 	shallow_nr += shallows.nr;
diff --git a/walker.c b/walker.c
index 2c86e406f9..274f1a4935 100644
--- a/walker.c
+++ b/walker.c
@@ -47,12 +47,12 @@ static int process_tree(struct walker *walker, struct tree *tree)
 		if (S_ISGITLINK(entry.mode))
 			continue;
 		if (S_ISDIR(entry.mode)) {
-			struct tree *tree = lookup_tree(entry.oid->hash);
+			struct tree *tree = lookup_tree(entry.oid);
 			if (tree)
 				obj = &tree->object;
 		}
 		else {
-			struct blob *blob = lookup_blob(entry.oid->hash);
+			struct blob *blob = lookup_blob(entry.oid);
 			if (blob)
 				obj = &blob->object;
 		}
@@ -180,7 +180,7 @@ static int loop(struct walker *walker)
 			}
 		}
 		if (!obj->type)
-			parse_object(obj->oid.hash);
+			parse_object(&obj->oid);
 		if (process_object(walker, obj))
 			return -1;
 	}
@@ -206,7 +206,7 @@ static int interpret_target(struct walker *walker, char *target, unsigned char *
 static int mark_complete(const char *path, const struct object_id *oid,
 			 int flag, void *cb_data)
 {
-	struct commit *commit = lookup_commit_reference_gently(oid->hash, 1);
+	struct commit *commit = lookup_commit_reference_gently(oid, 1);
 
 	if (commit) {
 		commit->object.flags |= COMPLETE;
diff --git a/wt-status.c b/wt-status.c
index f55f587111..5db53775ab 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -1428,7 +1428,7 @@ static void wt_status_get_detached_from(struct wt_status_state *state)
 	    /* sha1 is a commit? match without further lookup */
 	    (!oidcmp(&cb.noid, &oid) ||
 	     /* perhaps sha1 is a tag, try to dereference to a commit */
-	     ((commit = lookup_commit_reference_gently(oid.hash, 1)) != NULL &&
+	     ((commit = lookup_commit_reference_gently(&oid, 1)) != NULL &&
 	      !oidcmp(&cb.noid, &commit->object.oid)))) {
 		const char *from = ref;
 		if (!skip_prefix(from, "refs/tags/", &from))