diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index a11b46ce..f2f5b5c2 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -31,6 +31,7 @@
   "queens.ml": 2,
   "quicksort.ml": 44,
   "roman.ml": 44,
+  "rpn.ml": 9,
   "safe_div.ml": 20,
   "shuffle.ml": 55,
   "word_freq.ml": 8,
diff --git a/lib/ocaml/baseline/rpn.ml b/lib/ocaml/baseline/rpn.ml
new file mode 100644
index 00000000..d2fc7e94
--- /dev/null
+++ b/lib/ocaml/baseline/rpn.ml
@@ -0,0 +1,20 @@
+let eval_rpn tokens =
+  let stack = Stack.create () in
+  List.iter (fun tok ->
+    if tok = "+" || tok = "-" || tok = "*" || tok = "/" then begin
+      let b = Stack.pop stack in
+      let a = Stack.pop stack in
+      let r = if tok = "+" then a + b
+              else if tok = "-" then a - b
+              else if tok = "*" then a * b
+              else a / b
+      in
+      Stack.push r stack
+    end else
+      Stack.push (int_of_string tok) stack
+  ) tokens;
+  Stack.pop stack
+
+;;
+
+eval_rpn ["3"; "4"; "+"; "2"; "*"; "5"; "-"]
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index f9651bcc..a5c1e94f 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 — rpn.ml baseline (Reverse Polish Notation
+  evaluator using Stack). Walks the token list with List.iter, pushes
+  ints onto the stack, on operator tokens pops two operands and
+  pushes the result. `[3 4 + 2 * 5 -]` evaluates as 3+4=7, 7*2=14,
+  14-5=9 → 9. Exercises Stack.create / push / pop, mixed branch on
+  string equality, multi-arm if/else if, int_of_string for token
+  parsing. 39 baseline programs total.
 - 2026-05-09 Phase 5.1 — newton_sqrt.ml baseline (Newton's method
   for sqrt, sqrt(2)*1000 truncated → 1414). 20 iterations of
   `g := (g + x/g) / 2` converges to ~1.414213562 for x=2. Multiplied