diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 6a9c8c82..6b361776 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -122,6 +122,7 @@
   "quickselect.ml": 5,
   "quicksort.ml": 44,
   "roman.ml": 44,
+  "rolling_hash.ml": 6,
   "reverse_int.ml": 54329,
   "rpn.ml": 9,
   "run_decode.ml": 21,
diff --git a/lib/ocaml/baseline/rolling_hash.ml b/lib/ocaml/baseline/rolling_hash.ml
new file mode 100644
index 00000000..2b370cf9
--- /dev/null
+++ b/lib/ocaml/baseline/rolling_hash.ml
@@ -0,0 +1,38 @@
+let mod_p = 1000003
+let base = 257
+
+let rolling_match text pat =
+  let n = String.length text in
+  let m = String.length pat in
+  if m > n then 0
+  else begin
+    let pat_hash = ref 0 in
+    let win_hash = ref 0 in
+    let high = ref 1 in
+    for _ = 0 to m - 2 do
+      high := (!high * base) mod mod_p
+    done;
+    for i = 0 to m - 1 do
+      pat_hash := (!pat_hash * base + Char.code pat.[i]) mod mod_p;
+      win_hash := (!win_hash * base + Char.code text.[i]) mod mod_p
+    done;
+    let count = ref 0 in
+    for i = 0 to n - m do
+      if !win_hash = !pat_hash then begin
+        let ok = ref true in
+        for j = 0 to m - 1 do
+          if text.[i + j] <> pat.[j] then ok := false
+        done;
+        if !ok then count := !count + 1
+      end;
+      if i < n - m then begin
+        let drop = (Char.code text.[i] * !high) mod mod_p in
+        win_hash := ((!win_hash - drop + mod_p) * base + Char.code text.[i + m]) mod mod_p
+      end
+    done;
+    !count
+  end
+
+;;
+
+rolling_match "abcabcabcabcabcabc" "abc"
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index eb1d0725..97c7934e 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,15 @@ _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.
+- 2026-05-10 Phase 5.1 — rolling_hash.ml baseline (Rabin-Karp
+  rolling hash for substring matching, count "abc" in
+  "abcabcabcabcabcabc" = 6). Polynomial hash mod 1000003 with
+  base 257; precompute base^(m-1), then slide window updating
+  hash in O(1) per step. Verify hash matches with O(m) memcmp to
+  avoid false positives. Tests `for _ = 0 to m - 2 do … done`
+  unused loop variable, char-code arithmetic, mod under negative
+  intermediate, complex nested if/begin/end branching.
+  151 baseline programs total.
 - 2026-05-10 Phase 5.1 — huffman.ml baseline (Huffman tree weighted
   path length on letters {(5,a) (9,b) (12,c) (13,d) (16,e) (45,f)}
   = 224). Builds optimal prefix code by repeatedly merging the two