diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 95a8bbe7..62d779e8 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -58,6 +58,7 @@
   "safe_div.ml": 20,
   "shuffle.ml": 55,
   "subset_sum.ml": 8,
+  "tic_tac_toe.ml": 1,
   "word_freq.ml": 8,
   "zip_unzip.ml": 1000,
   "sieve.ml": 15,
diff --git a/lib/ocaml/baseline/tic_tac_toe.ml b/lib/ocaml/baseline/tic_tac_toe.ml
new file mode 100644
index 00000000..7eadef1f
--- /dev/null
+++ b/lib/ocaml/baseline/tic_tac_toe.ml
@@ -0,0 +1,34 @@
+let check_row b r =
+  let a = b.(r * 3) in
+  if a <> 0 && a = b.(r * 3 + 1) && a = b.(r * 3 + 2) then a
+  else 0
+
+let check_col b c =
+  let a = b.(c) in
+  if a <> 0 && a = b.(c + 3) && a = b.(c + 6) then a
+  else 0
+
+let check_diag b =
+  let a = b.(0) in
+  if a <> 0 && a = b.(4) && a = b.(8) then a
+  else
+    let b' = b.(2) in
+    if b' <> 0 && b' = b.(4) && b' = b.(6) then b'
+    else 0
+
+let winner b =
+  let r = ref 0 in
+  for i = 0 to 2 do
+    let cr = check_row b i in
+    if cr <> 0 then r := cr;
+    let cc = check_col b i in
+    if cc <> 0 then r := cc
+  done;
+  let cd = check_diag b in
+  if cd <> 0 then r := cd;
+  !r
+
+;;
+
+let b = Array.of_list [1;1;1; 0;2;0; 0;0;2] in
+winner b
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 880975f1..cd5033fb 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,13 @@ _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 — tic_tac_toe.ml baseline (3x3 winner check,
+  X wins top row → 1). Board encoded as 9-element flat int array
+  with 0=empty, 1=X, 2=O. Three predicate functions check row,
+  column, and either diagonal; main `winner` loops over the 8
+  winning lines. Tests Array.of_list with row-major indexing,
+  multi-fn collaboration, and structural equality on int values.
+  66 baseline programs total.
 - 2026-05-09 Phase 5.1 — subset_sum.ml baseline (count subsets of
   [1..8] summing to 10 = 8). Pure recursion: at each element, take
   it or don't. Base case: target=0 → 1, target≠0 → 0. 2^8 = 256