diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 98764167..f277cca4 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -4,6 +4,7 @@
   "exception_handle.ml": 4,
   "expr_eval.ml": 16,
   "factorial.ml": 3628800,
+  "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
   "module_use.ml": 3,
   "option_match.ml": 5,
diff --git a/lib/ocaml/baseline/fizzbuzz.ml b/lib/ocaml/baseline/fizzbuzz.ml
new file mode 100644
index 00000000..86933c01
--- /dev/null
+++ b/lib/ocaml/baseline/fizzbuzz.ml
@@ -0,0 +1,17 @@
+(* Baseline: fizzbuzz returning a list of strings *)
+let fizzbuzz n =
+  let acc = ref [] in
+  for i = 1 to n do
+    let s =
+      if i mod 15 = 0 then "FizzBuzz"
+      else if i mod 3 = 0 then "Fizz"
+      else if i mod 5 = 0 then "Buzz"
+      else Int.to_string i
+    in
+    acc := s :: !acc
+  done ;
+  List.rev !acc
+;;
+
+(* Concatenated for a deterministic check value via String.length *)
+String.length (String.concat "," (fizzbuzz 15))
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 6059f979..6e3dc860 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -399,6 +399,9 @@ _Newest first._
   recognise `!` as the prefix-deref of an application argument, so
   `String.concat "" (List.rev !b)` parses as `(... (deref b))`. Buffer
   uses a ref holding a string list; contents reverses and concats.
+- 2026-05-08 Phase 5.1 — fizzbuzz.ml baseline (12/12 pass). Classic
+  fizzbuzz using ref-cell accumulator, for-loop, mod, if/elseif chain,
+  String.concat, Int.to_string. Verifies output via String.length.
 - 2026-05-08 Phase 2+6 — print primitives wired to host `display` (+2
   tests, 444 total). `print_string` / `print_endline` / `print_int` /
   `print_newline` now use SX `display` (no auto-newline) plus an