diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 201762dd..3cce9ca5 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -25,6 +25,7 @@
   "expr_eval.ml": 16,
   "expr_simp.ml": 22,
   "factorial.ml": 3628800,
+  "fib_doubling.ml": 102334155,
   "fib_mod.ml": 391360,
   "fraction.ml": 7,
   "frequency.ml": 5,
diff --git a/lib/ocaml/baseline/fib_doubling.ml b/lib/ocaml/baseline/fib_doubling.ml
new file mode 100644
index 00000000..3843f459
--- /dev/null
+++ b/lib/ocaml/baseline/fib_doubling.ml
@@ -0,0 +1,14 @@
+let rec fib_pair n =
+  if n = 0 then (0, 1)
+  else
+    let (a, b) = fib_pair (n / 2) in
+    let c = a * (2 * b - a) in
+    let d = a * a + b * b in
+    if n mod 2 = 0 then (c, d)
+    else (d, c + d)
+
+let fib n = let (f, _) = fib_pair n in f
+
+;;
+
+fib 40
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index f3ade729..f80483a2 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 — fib_doubling.ml baseline (Fibonacci by
+  doubling, fib(40) = 102334155). Uses the identity F(2k) = F(k) *
+  (2*F(k+1) - F(k)) and F(2k+1) = F(k)^2 + F(k+1)^2 to compute fib
+  in O(log n) recursive depth. Returns a tuple (F(n), F(n+1)) at
+  each step. fib(40) = 102334155 fits in JS safe-int (< 2^53). 86
+  baseline programs total.
 - 2026-05-09 Phase 5.1 — merge_two.ml baseline (merge two sorted
   lists, length*sum = 9*49 = 441). Standard two-finger merge with
   nested match-in-match. Used as a building block in merge_sort.ml