diff --git a/lib/ocaml/baseline/euler7.ml b/lib/ocaml/baseline/euler7.ml
new file mode 100644
index 00000000..e4ad4b1e
--- /dev/null
+++ b/lib/ocaml/baseline/euler7.ml
@@ -0,0 +1,22 @@
+let nth_prime n =
+  let count = ref 0 in
+  let i = ref 1 in
+  let result = ref 0 in
+  while !count < n do
+    i := !i + 1;
+    let p = ref true in
+    let j = ref 2 in
+    while !j * !j <= !i && !p do
+      if !i mod !j = 0 then p := false;
+      j := !j + 1
+    done;
+    if !p then begin
+      count := !count + 1;
+      if !count = n then result := !i
+    end
+  done;
+  !result
+
+;;
+
+nth_prime 100
diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index e9824aa4..1db73d94 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -31,6 +31,7 @@
   "euler4_small.ml": 9009,
   "euler5.ml": 232792560,
   "euler6.ml": 25164150,
+  "euler7.ml": 541,
   "euler9.ml": 31875000,
   "expr_eval.ml": 16,
   "expr_simp.ml": 22,
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 5c1d1f87..1f39a248 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,11 @@ _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 — euler7.ml baseline (100th prime = 541;
+  scaled-down PE7 which asks for the 10001st = 104743). Trial-
+  division within an outer while loop searching forward from 2,
+  short-circuited via bool ref. 100 keeps under 3-min budget while
+  exercising the same algorithm. 112 baseline programs total.
 - 2026-05-09 Phase 5.1 — euler4_small.ml baseline (largest 2-digit
   palindrome product = 9009 = 91 * 99). Scaled-down Project Euler
   #4 (real version uses 3-digit numbers, 906609; that's 810k inner