diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 1b08ac96..3453f55b 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -31,6 +31,7 @@
   "roman.ml": 44,
   "safe_div.ml": 20,
   "shuffle.ml": 55,
+  "word_freq.ml": 8,
   "sieve.ml": 15,
   "sum_squares.ml": 385,
   "unique_set.ml": 9,
diff --git a/lib/ocaml/baseline/word_freq.ml b/lib/ocaml/baseline/word_freq.ml
new file mode 100644
index 00000000..13dcac56
--- /dev/null
+++ b/lib/ocaml/baseline/word_freq.ml
@@ -0,0 +1,21 @@
+module StringOrd = struct
+  type t = string
+  let compare = String.compare
+end
+
+module SMap = Map.Make (StringOrd)
+
+let count_words text =
+  let words = String.split_on_char ' ' text in
+  List.fold_left (fun m w ->
+    let n = match SMap.find_opt w m with
+            | Some n -> n
+            | None -> 0
+    in
+    SMap.add w (n + 1) m
+  ) SMap.empty words
+
+;;
+
+let m = count_words "the quick brown fox jumps over the lazy dog" in
+SMap.cardinal m
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 51911fb1..4f7654f3 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 — word_freq.ml baseline (Map.Make on String,
+  count distinct words → 8). Defines a StringOrd module + applies
+  Map.Make to it. Folds the input through SMap.find_opt + SMap.add to
+  count each word, then reports SMap.cardinal. "the quick brown fox
+  jumps over the lazy dog" — "the" appears twice, so 8 distinct
+  words. First baseline using Map.Make on a string-keyed map. 35
+  baseline programs total.
 - 2026-05-09 Phase 6 — Either module + Hashtbl.copy (+4 tests, 602
   total). Either: left, right, is_left, is_right, find_left,
   find_right, map_left, map_right, fold, equal, compare. Constructors