diff --git a/lib/ocaml/baseline/euler1.ml b/lib/ocaml/baseline/euler1.ml
new file mode 100644
index 00000000..42cb2d01
--- /dev/null
+++ b/lib/ocaml/baseline/euler1.ml
@@ -0,0 +1,10 @@
+let euler1 limit =
+  let sum = ref 0 in
+  for i = 1 to limit - 1 do
+    if i mod 3 = 0 || i mod 5 = 0 then sum := !sum + i
+  done;
+  !sum
+
+;;
+
+euler1 1000
diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index bc8170b1..dd452b56 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -24,6 +24,7 @@
   "csv.ml": 10,
   "exception_handle.ml": 4,
   "exception_user.ml": 26,
+  "euler1.ml": 233168,
   "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 3c78bdef..66b49f0d 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,9 @@ _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 — euler1.ml baseline (Project Euler #1, sum
+  of multiples of 3 or 5 below 1000 = 233168). Trivial DSL exercise
+  but symbolically meaningful: this is the 100th baseline program.
 - 2026-05-09 Phase 5.1 — anagram_groups.ml baseline (group strings
   by canonical anagram form, ["eat";"tea";"tan";"ate";"nat";"bat"]
   has 3 groups). canonical builds a sorted-by-frequency string