diff --git a/builtin/merge.c b/builtin/merge.c
index 9d5359edc2..87dfc9bc06 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -28,6 +28,7 @@
 #include "rerere.h"
 #include "help.h"
 #include "merge-recursive.h"
+#include "merge-ort-wrappers.h"
 #include "resolve-undo.h"
 #include "remote.h"
 #include "fmt-merge-msg.h"
@@ -88,6 +89,7 @@ static int no_verify;
 static struct strategy all_strategy[] = {
 	{ "recursive",  DEFAULT_TWOHEAD | NO_TRIVIAL },
 	{ "octopus",    DEFAULT_OCTOPUS },
+	{ "ort",        NO_TRIVIAL },
 	{ "resolve",    0 },
 	{ "ours",       NO_FAST_FORWARD | NO_TRIVIAL },
 	{ "subtree",    NO_FAST_FORWARD | NO_TRIVIAL },
@@ -159,10 +161,17 @@ static struct strategy *get_strategy(const char *name)
 	struct strategy *ret;
 	static struct cmdnames main_cmds, other_cmds;
 	static int loaded;
+	char *default_strategy = getenv("GIT_TEST_MERGE_ALGORITHM");
 
 	if (!name)
 		return NULL;
 
+	if (default_strategy &&
+	    !strcmp(default_strategy, "ort") &&
+	    !strcmp(name, "recursive")) {
+		name = "ort";
+	}
+
 	for (i = 0; i < ARRAY_SIZE(all_strategy); i++)
 		if (!strcmp(name, all_strategy[i].name))
 			return &all_strategy[i];
@@ -701,7 +710,8 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
 	if (refresh_and_write_cache(REFRESH_QUIET, SKIP_IF_UNCHANGED, 0) < 0)
 		return error(_("Unable to write index."));
 
-	if (!strcmp(strategy, "recursive") || !strcmp(strategy, "subtree")) {
+	if (!strcmp(strategy, "recursive") || !strcmp(strategy, "subtree") ||
+	    !strcmp(strategy, "ort")) {
 		struct lock_file lock = LOCK_INIT;
 		int clean, x;
 		struct commit *result;
@@ -732,8 +742,12 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
 			commit_list_insert(j->item, &reversed);
 
 		hold_locked_index(&lock, LOCK_DIE_ON_ERROR);
-		clean = merge_recursive(&o, head,
-				remoteheads->item, reversed, &result);
+		if (!strcmp(strategy, "ort"))
+			clean = merge_ort_recursive(&o, head, remoteheads->item,
+						    reversed, &result);
+		else
+			clean = merge_recursive(&o, head, remoteheads->item,
+						reversed, &result);
 		if (clean < 0)
 			exit(128);
 		if (write_locked_index(&the_index, &lock,
@@ -1264,6 +1278,12 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	if (branch)
 		skip_prefix(branch, "refs/heads/", &branch);
 
+	if (!pull_twohead) {
+		char *default_strategy = getenv("GIT_TEST_MERGE_ALGORITHM");
+		if (default_strategy && !strcmp(default_strategy, "ort"))
+			pull_twohead = "ort";
+	}
+
 	init_diff_ui_defaults();
 	git_config(git_merge_config, NULL);
 
diff --git a/builtin/rebase.c b/builtin/rebase.c
index eeca53382f..9719aa25da 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -119,6 +119,7 @@ static struct replay_opts get_replay_opts(const struct rebase_options *opts)
 	struct replay_opts replay = REPLAY_OPTS_INIT;
 
 	replay.action = REPLAY_INTERACTIVE_REBASE;
+	replay.strategy = NULL;
 	sequencer_init_config(&replay);
 
 	replay.signoff = opts->signoff;
@@ -136,7 +137,8 @@ static struct replay_opts get_replay_opts(const struct rebase_options *opts)
 					opts->committer_date_is_author_date;
 	replay.ignore_date = opts->ignore_date;
 	replay.gpg_sign = xstrdup_or_null(opts->gpg_sign_opt);
-	replay.strategy = opts->strategy;
+	if (opts->strategy)
+		replay.strategy = opts->strategy;
 
 	if (opts->strategy_opts)
 		parse_strategy_opts(&replay, opts->strategy_opts);
@@ -1771,6 +1773,11 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			    options.default_backend);
 	}
 
+	if (options.type == REBASE_MERGE &&
+	    !options.strategy &&
+	    getenv("GIT_TEST_MERGE_ALGORITHM"))
+		options.strategy = xstrdup(getenv("GIT_TEST_MERGE_ALGORITHM"));
+
 	switch (options.type) {
 	case REBASE_MERGE:
 	case REBASE_PRESERVE_MERGES:
diff --git a/builtin/revert.c b/builtin/revert.c
index f61cc5d82c..c7cb0c1a18 100644
--- a/builtin/revert.c
+++ b/builtin/revert.c
@@ -202,6 +202,8 @@ static int run_sequencer(int argc, const char **argv, struct replay_opts *opts)
 	/* These option values will be free()d */
 	opts->gpg_sign = xstrdup_or_null(opts->gpg_sign);
 	opts->strategy = xstrdup_or_null(opts->strategy);
+	if (!opts->strategy && getenv("GIT_TEST_MERGE_ALGORITHM"))
+		opts->strategy = xstrdup(getenv("GIT_TEST_MERGE_ALGORITHM"));
 
 	if (cmd == 'q') {
 		int ret = sequencer_remove_state(opts);
diff --git a/sequencer.c b/sequencer.c
index 00acb12496..e16d85380c 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -14,7 +14,8 @@
 #include "diff.h"
 #include "revision.h"
 #include "rerere.h"
-#include "merge-recursive.h"
+#include "merge-ort.h"
+#include "merge-ort-wrappers.h"
 #include "refs.h"
 #include "strvec.h"
 #include "quote.h"
@@ -204,6 +205,20 @@ static int git_sequencer_config(const char *k, const char *v, void *cb)
 		return 0;
 	}
 
+	if (!opts->strategy && !strcmp(k, "pull.twohead")) {
+		int ret = git_config_string((const char**)&opts->strategy, k, v);
+		if (ret == 0) {
+			/*
+			 * pull.twohead is allowed to be multi-valued; we only
+			 * care about the first value.
+			 */
+			char *tmp = strchr(opts->strategy, ' ');
+			if (tmp)
+				*tmp = '\0';
+		}
+		return ret;
+	}
+
 	status = git_gpg_config(k, v, NULL);
 	if (status)
 		return status;
@@ -595,8 +610,9 @@ static int do_recursive_merge(struct repository *r,
 			      struct replay_opts *opts)
 {
 	struct merge_options o;
+	struct merge_result result;
 	struct tree *next_tree, *base_tree, *head_tree;
-	int clean;
+	int clean, show_output;
 	int i;
 	struct lock_file index_lock = LOCK_INIT;
 
@@ -620,12 +636,27 @@ static int do_recursive_merge(struct repository *r,
 	for (i = 0; i < opts->xopts_nr; i++)
 		parse_merge_opt(&o, opts->xopts[i]);
 
-	clean = merge_trees(&o,
-			    head_tree,
-			    next_tree, base_tree);
-	if (is_rebase_i(opts) && clean <= 0)
-		fputs(o.obuf.buf, stdout);
-	strbuf_release(&o.obuf);
+	if (opts->strategy && !strcmp(opts->strategy, "ort")) {
+		memset(&result, 0, sizeof(result));
+		merge_incore_nonrecursive(&o, base_tree, head_tree, next_tree,
+					    &result);
+		show_output = !is_rebase_i(opts) || !result.clean;
+		/*
+		 * TODO: merge_switch_to_result will update index/working tree;
+		 * we only really want to do that if !result.clean || this is
+		 * the final patch to be picked.  But determining this is the
+		 * final patch would take some work, and "head_tree" would need
+		 * to be replace with the tree the index matched before we
+		 * started doing any picks.
+		 */
+		merge_switch_to_result(&o, head_tree, &result, 1, show_output);
+		clean = result.clean;
+	} else {
+		clean = merge_trees(&o, head_tree, next_tree, base_tree);
+		if (is_rebase_i(opts) && clean <= 0)
+			fputs(o.obuf.buf, stdout);
+		strbuf_release(&o.obuf);
+	}
 	if (clean < 0) {
 		rollback_lock_file(&index_lock);
 		return clean;
@@ -1991,7 +2022,10 @@ static int do_pick_commit(struct repository *r,
 
 	if (is_rebase_i(opts) && write_author_script(msg.message) < 0)
 		res = -1;
-	else if (!opts->strategy || !strcmp(opts->strategy, "recursive") || command == TODO_REVERT) {
+	else if (!opts->strategy ||
+		 !strcmp(opts->strategy, "recursive") ||
+		 !strcmp(opts->strategy, "ort") ||
+		 command == TODO_REVERT) {
 		res = do_recursive_merge(r, base, next, base_label, next_label,
 					 &head, &msgbuf, opts);
 		if (res < 0)
@@ -3485,7 +3519,9 @@ static int do_merge(struct repository *r,
 	struct commit_list *bases, *j, *reversed = NULL;
 	struct commit_list *to_merge = NULL, **tail = &to_merge;
 	const char *strategy = !opts->xopts_nr &&
-		(!opts->strategy || !strcmp(opts->strategy, "recursive")) ?
+		(!opts->strategy ||
+		 !strcmp(opts->strategy, "recursive") ||
+		 !strcmp(opts->strategy, "ort")) ?
 		NULL : opts->strategy;
 	struct merge_options o;
 	int merge_arg_len, oneline_offset, can_fast_forward, ret, k;
@@ -3722,7 +3758,20 @@ static int do_merge(struct repository *r,
 	o.branch2 = ref_name.buf;
 	o.buffer_output = 2;
 
-	ret = merge_recursive(&o, head_commit, merge_commit, reversed, &i);
+	if (opts->strategy && !strcmp(opts->strategy, "ort")) {
+		/*
+		 * TODO: Should use merge_incore_recursive() and
+		 * merge_switch_to_result(), skipping the call to
+		 * merge_switch_to_result() when we don't actually need to
+		 * update the index and working copy immediately.
+		 */
+		ret = merge_ort_recursive(&o,
+					  head_commit, merge_commit, reversed,
+					  &i);
+	} else {
+		ret = merge_recursive(&o, head_commit, merge_commit, reversed,
+				      &i);
+	}
 	if (ret <= 0)
 		fputs(o.obuf.buf, stdout);
 	strbuf_release(&o.obuf);
