diff --git a/lib/ocaml/baseline/euler6.ml b/lib/ocaml/baseline/euler6.ml
new file mode 100644
index 00000000..d8d8cf5c
--- /dev/null
+++ b/lib/ocaml/baseline/euler6.ml
@@ -0,0 +1,12 @@
+let euler6 n =
+  let sum = ref 0 in
+  let sum_sq = ref 0 in
+  for i = 1 to n do
+    sum := !sum + i;
+    sum_sq := !sum_sq + i * i
+  done;
+  !sum * !sum - !sum_sq
+
+;;
+
+euler6 100
diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 146a53ce..5285bebd 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -26,6 +26,7 @@
   "exception_user.ml": 26,
   "euler1.ml": 233168,
   "euler2.ml": 4613732,
+  "euler6.ml": 25164150,
   "expr_eval.ml": 16,
   "expr_simp.ml": 22,
   "factorial.ml": 3628800,
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 5596f6bc..253e4b2b 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,10 @@ _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 — euler6.ml baseline (Project Euler #6, square
+  of sum minus sum of squares for 1..100 = 25164150). Single for-loop
+  threading two refs; (sum 1..100)^2 - sum(i^2 for 1..100) = 5050^2
+  - 338350 = 25502500 - 338350 = 25164150. 102 baseline programs total.
 - 2026-05-09 Phase 5.1 — euler2.ml baseline (Project Euler #2, sum
   of even Fibonacci ≤ 4M = 4613732). Iterative two-ref Fibonacci,
   accumulating only even terms. Sequence: 1, 2, 3, 5, 8, 13, 21,