diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 805525cc..469e8e6c 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -52,6 +52,7 @@
   "mutable_record.ml": 10,
   "option_match.ml": 5,
   "palindrome.ml": 4,
+  "paren_depth.ml": 7,
   "pancake_sort.ml": 910,
   "pascal.ml": 252,
   "pi_leibniz.ml": 314,
diff --git a/lib/ocaml/baseline/paren_depth.ml b/lib/ocaml/baseline/paren_depth.ml
new file mode 100644
index 00000000..41e4648b
--- /dev/null
+++ b/lib/ocaml/baseline/paren_depth.ml
@@ -0,0 +1,15 @@
+let max_depth s =
+  let d = ref 0 in
+  let m = ref 0 in
+  for i = 0 to String.length s - 1 do
+    if s.[i] = '(' then begin
+      d := !d + 1;
+      if !d > !m then m := !d
+    end
+    else if s.[i] = ')' then d := !d - 1
+  done;
+  !m
+
+;;
+
+max_depth "((1+2)*(3-(4+5)))" + max_depth "(((deep)))" + max_depth "()()()"
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 615a22a8..a02864ac 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,14 @@ _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 — paren_depth.ml baseline (max paren nesting
+  depth, 3+3+1 = 7). One-pass walk tracking current depth and a
+  high-water mark. Tests three inputs:
+    "((1+2)*(3-(4+5)))" → 3
+    "(((deep)))"        → 3
+    "()()()"            → 1
+  Sum = 7. Tests for-loop char comparison `s.[i] = '('` and
+  high-water-mark idiom with two refs. 78 baseline programs total.
 - 2026-05-09 Phase 5.1 — pancake_sort.ml baseline (in-place pancake
   sort, 9 flips → 910). Each pass finds the max in [0..size-1],
   flips it to position 0 (if needed), then flips the size-prefix to