diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 2f291158..140edeea 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -20,6 +20,7 @@
   "factorial.ml": 3628800,
   "fraction.ml": 7,
   "frequency.ml": 5,
+  "gcd_lcm.ml": 60,
   "grep_count.ml": 3,
   "hanoi.ml": 1023,
   "fizzbuzz.ml": 57,
diff --git a/lib/ocaml/baseline/gcd_lcm.ml b/lib/ocaml/baseline/gcd_lcm.ml
new file mode 100644
index 00000000..5b958db0
--- /dev/null
+++ b/lib/ocaml/baseline/gcd_lcm.ml
@@ -0,0 +1,6 @@
+let rec gcd a b = if b = 0 then a else gcd b (a mod b)
+
+let lcm a b = a * b / gcd a b
+;;
+
+gcd 36 48 + lcm 4 6 + lcm 12 18
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 82eee160..377f0165 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,12 @@ _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 — gcd_lcm.ml baseline (Euclidean gcd + lcm,
+  12 + 12 + 36 = 60). Two-line baseline: `let rec gcd a b = if b = 0
+  then a else gcd b (a mod b)` + `let lcm a b = a * b / gcd a b`.
+  Tests `mod` arithmetic and the integer-division fix from
+  iteration 94 (without truncate-toward-zero, lcm 4 6 = 4*6 / 2 =
+  12.0 not 12). 54 baseline programs total.
 - 2026-05-09 Phase 5.1 — zip_unzip.ml baseline (list zip/unzip
   round-trip, sum-product = 1000). zip walks both lists in lockstep
   truncating at the shorter; unzip uses tuple-pattern destructuring