diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index b97ffddf..35b76e82 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -22,6 +22,7 @@
   "hanoi.ml": 1023,
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
+  "mat_mul.ml": 621,
   "json_pretty.ml": 24,
   "kadane.ml": 6,
   "lambda_calc.ml": 7,
diff --git a/lib/ocaml/baseline/mat_mul.ml b/lib/ocaml/baseline/mat_mul.ml
new file mode 100644
index 00000000..e7593f4f
--- /dev/null
+++ b/lib/ocaml/baseline/mat_mul.ml
@@ -0,0 +1,18 @@
+let mat_mul a b n =
+  let c = Array.make (n * n) 0 in
+  for i = 0 to n - 1 do
+    for j = 0 to n - 1 do
+      for k = 0 to n - 1 do
+        c.(i * n + j) <- c.(i * n + j) + a.(i * n + k) * b.(k * n + j)
+      done
+    done
+  done;
+  c
+
+;;
+
+let n = 3 in
+let a = Array.of_list [1;2;3; 4;5;6; 7;8;9] in
+let b = Array.of_list [9;8;7; 6;5;4; 3;2;1] in
+let c = mat_mul a b n in
+Array.fold_left (+) 0 c
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 5a516692..68e5a959 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 — mat_mul.ml baseline (3x3 row-major matrix
+  multiply, sum of result = 621). Triple-nested for loop over
+  i / j / k with row-major indexing `c.(i * n + j) <- c.(i * n + j) +
+  a.(i * n + k) * b.(k * n + j)`. Tests deeply nested for loops on
+  Array, Array.make + arr.(i) + arr.(i) <- v + Array.fold_left, and
+  multi-arg let chains with intermediate Array bindings. 50 baseline
+  programs total — milestone.
 - 2026-05-09 Phase 5.1 — bsearch.ml baseline (binary search,
   position-sum 6 + 2 + (-1) = 7). Iterative bsearch on a sorted int
   array using two index refs `lo`/`hi` and a sentinel `found = -1`.