diff --git a/lib/ocaml/parser.sx b/lib/ocaml/parser.sx
index 459e4fc2..bd2b77d8 100644
--- a/lib/ocaml/parser.sx
+++ b/lib/ocaml/parser.sx
@@ -353,6 +353,15 @@
                   (advance-tok!)
                   (set-nth! wild-counter 0 (+ (nth wild-counter 0) 1))
                   (str "__wild_" (nth wild-counter 0))))
+              ;; ~name / ?name labeled/optional param — drop the label
+              ;; prefix and treat as a positional ident name. Optional
+              ;; default `?(name = default)` is not handled here yet.
+              ((and (or (at-op? "~") (at-op? "?"))
+                    (= (ocaml-tok-type (peek-tok-at 1)) "ident"))
+                (begin
+                  (advance-tok!) ;; ~ or ?
+                  (let ((nm (ocaml-tok-value (peek-tok))))
+                    (begin (advance-tok!) nm))))
               ((check-tok? "ident" nil)
                 (let ((nm (ocaml-tok-value (peek-tok))))
                   (begin (advance-tok!) nm)))
@@ -565,6 +574,9 @@
                 ((and (= tt "keyword") (or (= tv "true") (= tv "false") (= tv "begin")))
                   true)
                 ((and (= tt "op") (or (= tv "(") (= tv "[") (= tv "{") (= tv "!"))) true)
+                ;; ~name or ?name (labeled/optional arg) — drop the
+                ;; label prefix and use the value as a positional arg.
+                ((and (= tt "op") (or (= tv "~") (= tv "?"))) true)
                 (else false)))))
         (define parse-atom-postfix
           (fn ()
@@ -637,6 +649,20 @@
                             ((at-op? "!")
                               (begin (advance-tok!)
                                      (list :deref (parse-atom-postfix))))
+                            ;; Labeled/optional arg ~name[:VAL] or ?name[:VAL]
+                            ;; — drop the label prefix. With colon, parse VAL
+                            ;; as the arg. Without, the variable named `name`
+                            ;; (punning) becomes the arg.
+                            ((or (at-op? "~") (at-op? "?"))
+                              (begin
+                                (advance-tok!)
+                                (let ((nm (ocaml-tok-value
+                                            (consume! "ident" nil))))
+                                  (cond
+                                    ((at-op? ":")
+                                      (begin (advance-tok!)
+                                             (parse-atom-postfix)))
+                                    (else (list :var nm))))))
                             (else (parse-atom-postfix)))))
                         (begin (set! head (list :app head arg)) (loop))))))
                 (loop)
diff --git a/lib/ocaml/test.sh b/lib/ocaml/test.sh
index 2834ef4c..8723d255 100755
--- a/lib/ocaml/test.sh
+++ b/lib/ocaml/test.sh
@@ -1396,6 +1396,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 5142)
 (eval "(ocaml-run-program \"let h (a, b) (c, d) = a * b + c * d ;; h (1, 2) (3, 4)\")")
 
+;; ── Labeled / optional args (label dropped) ──────────────────
+(epoch 5150)
+(eval "(ocaml-run \"let f ~x ~y = x + y in f ~x:3 ~y:7\")")
+(epoch 5151)
+(eval "(ocaml-run \"let f ~x ~y = x * y in let x = 4 in let y = 5 in f ~x ~y\")")
+(epoch 5152)
+(eval "(ocaml-run \"let f ?x ~y = x + y in f ?x:1 ~y:2\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -2219,6 +2227,11 @@ check 5140 "top let f (a, b) = a+b"       '10'
 check 5141 "top let g x (a, b) mixed"     '6'
 check 5142 "top let h (a, b) (c, d)"      '14'
 
+# ── Labeled / optional args (label dropped, positional) ─────────
+check 5150 "f ~x:3 ~y:7 sum"              '10'
+check 5151 "f ~x ~y punning"              '20'
+check 5152 "f ?x:1 ~y:2 (no Some wrap)"   '3'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 17e0c98c..7a4264be 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,20 @@ _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 4 — basic labeled / optional argument syntax
+  (label dropped, positional semantics) (+3 tests, 562 total). Three
+  parser changes:
+  (1) `at-app-start?` returns true on op `~` or `?` so the app loop
+      keeps consuming labeled args;
+  (2) the app arg parser handles `~name:VAL` (drop label, parse VAL),
+      `?name:VAL` (same), and `~name` punning (treat as `(:var name)`);
+  (3) `try-consume-param!` drops `~` / `?` and treats the following
+      ident as a regular positional param name.
+  Order in the call must match definition order — we don't reorder
+  args by label name. Optional args don't auto-wrap in Some, so the
+  function body sees the raw value for `?x:V`. Lets us write
+  `let f ~x ~y = x + y in f ~x:3 ~y:7` and `let x = 4 in let y = 5
+  in f ~x ~y` (punning).
 - 2026-05-09 Phase 5.1 — merge_sort.ml baseline (user-implemented
   mergesort, sorted sum = 44). Stress-tests `let (a, b) = split rest
   in (x :: a, y :: b)` (let-tuple destructuring inside a recursive