diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 4d72e60d..208228be 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -61,6 +61,7 @@
   "subset_sum.ml": 8,
   "tic_tac_toe.ml": 1,
   "word_freq.ml": 8,
+  "zigzag.ml": 55,
   "zip_unzip.ml": 1000,
   "sieve.ml": 15,
   "sum_squares.ml": 385,
diff --git a/lib/ocaml/baseline/zigzag.ml b/lib/ocaml/baseline/zigzag.ml
new file mode 100644
index 00000000..710644f0
--- /dev/null
+++ b/lib/ocaml/baseline/zigzag.ml
@@ -0,0 +1,8 @@
+let rec zigzag xs ys =
+  match xs with
+  | [] -> ys
+  | x :: xs' -> x :: zigzag ys xs'
+
+;;
+
+List.fold_left (+) 0 (zigzag [1;3;5;7;9] [2;4;6;8;10])
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 153ed59f..09bcf0a2 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 — zigzag.ml baseline (interleave two lists,
+  sum 1..10 = 55). One-liner that swaps the lists on every recursive
+  call: `match xs with [] -> ys | x :: xs' -> x :: zigzag ys xs'`.
+  zigzag [1;3;5;7;9] [2;4;6;8;10] = [1;2;3;4;5;6;7;8;9;10] sum 55.
+  Tests recursive list cons + arg-swap idiom. 68 baseline programs
+  total.
 - 2026-05-09 Phase 5.1 — prefix_sum.ml baseline (precomputed prefix
   sums for O(1) range queries, sum of three queries = 66).
   prefix_sums xs returns an Array of len n+1 such that