diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 33932e5d..025653bd 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -196,6 +196,7 @@
   "triangle_div.ml": 120,
   "twosum.ml": 5,
   "union_find.ml": 4,
+  "unique_paths_obs.ml": 3,
   "unique_set.ml": 9,
   "validate.ml": 417,
   "word_count.ml": 3
diff --git a/lib/ocaml/baseline/unique_paths_obs.ml b/lib/ocaml/baseline/unique_paths_obs.ml
new file mode 100644
index 00000000..1a3db496
--- /dev/null
+++ b/lib/ocaml/baseline/unique_paths_obs.ml
@@ -0,0 +1,28 @@
+let h = 4
+let w = 4
+let grid = [|
+  [| 0; 0; 0; 0 |];
+  [| 0; 1; 0; 0 |];
+  [| 0; 0; 0; 1 |];
+  [| 1; 0; 0; 0 |]
+|]
+
+let paths_with_obs () =
+  let dp = Array.init h (fun _ -> Array.make w 0) in
+  if grid.(0).(0) = 0 then dp.(0).(0) <- 1;
+  for j = 1 to w - 1 do
+    if grid.(0).(j) = 0 then dp.(0).(j) <- dp.(0).(j - 1)
+  done;
+  for i = 1 to h - 1 do
+    if grid.(i).(0) = 0 then dp.(i).(0) <- dp.(i - 1).(0)
+  done;
+  for i = 1 to h - 1 do
+    for j = 1 to w - 1 do
+      if grid.(i).(j) = 0 then
+        dp.(i).(j) <- dp.(i - 1).(j) + dp.(i).(j - 1)
+    done
+  done;
+  dp.(h - 1).(w - 1)
+;;
+
+paths_with_obs ()
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index defa645b..50b4dedc 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.
+- 2026-05-11 Phase 5.1 — unique_paths_obs.ml baseline (count
+  monotone paths in 4×4 grid with obstacles at (1,1),(2,3),(3,0)
+  = 3). Standard 2D DP with obstacle gating: dp[i][j] = dp[i-1][j]
+  + dp[i][j-1] when grid[i][j]=0, else 0. Complements grid_paths.ml
+  (no-obstacles version): the same DP but obstacles zero out
+  cells, reshaping the path count. 199 baseline programs total.
 - 2026-05-11 Phase 5.1 — daily_temperatures.ml baseline (monotonic
   decreasing stack of waiting days until warmer; sum over
   [73;74;75;71;69;72;76;73] = 10). For each day i, pop all stack