diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 2601a34b..b767248a 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -80,6 +80,7 @@
   "subset_sum.ml": 8,
   "tic_tac_toe.ml": 1,
   "word_freq.ml": 8,
+  "xor_cipher.ml": 601,
   "zigzag.ml": 55,
   "zip_unzip.ml": 1000,
   "sieve.ml": 15,
diff --git a/lib/ocaml/baseline/xor_cipher.ml b/lib/ocaml/baseline/xor_cipher.ml
new file mode 100644
index 00000000..61ca6775
--- /dev/null
+++ b/lib/ocaml/baseline/xor_cipher.ml
@@ -0,0 +1,16 @@
+let xor_cipher key text =
+  let n = String.length text in
+  let kn = String.length key in
+  let buf = Buffer.create n in
+  for i = 0 to n - 1 do
+    let c = Char.code text.[i] in
+    let k = Char.code key.[i mod kn] in
+    Buffer.add_string buf (String.make 1 (Char.chr (c lxor k)))
+  done;
+  Buffer.contents buf
+
+;;
+
+let encoded = xor_cipher "key" "Hello!" in
+let decoded = xor_cipher "key" encoded in
+String.length decoded * 100 + (if decoded = "Hello!" then 1 else 0)
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 10c89eb5..3e1b5c6f 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 — xor_cipher.ml baseline (XOR roll-key
+  encryption, round-trip → 601). For each character, XOR with the
+  corresponding key char (key cycled via `i mod kn`). Encrypts
+  "Hello!" with key "key", decrypts the result, and verifies the
+  round-trip preserves both length (6) and equality. Tests
+  Char.code + Char.chr round-trip + the iter-127 `lxor` operator
+  + Buffer.add_string + String.make 1. 90 baseline programs total.
 - 2026-05-09 Phase 5.1 — simpson_int.ml baseline (Simpson's rule
   numerical integration, ∫₀¹ x² dx ≈ 1/3, scaled = 10000). Composite
   Simpson's 1/3 rule with 100 panels. Coefficients 1-4-2-...-2-4-1