diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 9788d392..c6706fe0 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -60,6 +60,7 @@
   "zip_unzip.ml": 1000,
   "sieve.ml": 15,
   "sum_squares.ml": 385,
+  "twosum.ml": 5,
   "unique_set.ml": 9,
   "validate.ml": 417,
   "word_count.ml": 3
diff --git a/lib/ocaml/baseline/twosum.ml b/lib/ocaml/baseline/twosum.ml
new file mode 100644
index 00000000..a6011476
--- /dev/null
+++ b/lib/ocaml/baseline/twosum.ml
@@ -0,0 +1,17 @@
+let twosum xs target =
+  let h = Hashtbl.create 8 in
+  let result = ref (-1, -1) in
+  List.iteri (fun i x ->
+    let need = target - x in
+    match Hashtbl.find_opt h need with
+    | Some j -> result := (j, i)
+    | None -> Hashtbl.add h x i
+  ) xs;
+  !result
+
+;;
+
+let (i1, j1) = twosum [2;7;11;15] 9 in
+let (i2, j2) = twosum [3;2;4] 6 in
+let (i3, j3) = twosum [3;3] 6 in
+i1 + j1 + i2 + j2 + i3 + j3
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index acf3e929..9eee200c 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,17 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 5.1 — twosum.ml baseline (LeetCode #1, hashtable
+  one-pass, index-sum 1+3+1 = 5). Walks list with List.iteri,
+  checking if `target - x` is already in the hashtable; if yes, the
+  earlier index plus current is the answer; otherwise record the
+  current pair. Three test cases:
+    [2;7;11;15] target 9 → (0, 1)
+    [3;2;4]     target 6 → (1, 2)
+    [3;3]       target 6 → (0, 1)
+  Sum of i+j over each pair: 1+3+1 = 5. Tests Hashtbl.find_opt /
+  add + List.iteri + tuple destructuring on let-binding (iter 98).
+  63 baseline programs total.
 - 2026-05-09 Phase 5.1 — bisect.ml baseline (root-finding via
   bisection, sqrt(2) * 100 = 141). 50 iterations of bisection
   searching for x^2 - 2 = 0 in [1, 2]. Tests higher-order function