diff --git a/lib/ocaml/parser.sx b/lib/ocaml/parser.sx
index cf71cad7..0ff699d7 100644
--- a/lib/ocaml/parser.sx
+++ b/lib/ocaml/parser.sx
@@ -167,7 +167,7 @@
                 ((= tt "ctor") true)
                 ((and (= tt "keyword") (or (= tv "true") (= tv "false")))
                   true)
-                ((and (= tt "op") (or (= tv "(") (= tv "[") (= tv "{"))) true)
+                ((and (= tt "op") (or (= tv "(") (= tv "[") (= tv "{") (= tv "!"))) true)
                 (else false)))))
         (set!
           parse-pattern-atom
@@ -532,7 +532,7 @@
                 ((= tt "ctor") true)
                 ((and (= tt "keyword") (or (= tv "true") (= tv "false") (= tv "begin")))
                   true)
-                ((and (= tt "op") (or (= tv "(") (= tv "[") (= tv "{"))) true)
+                ((and (= tt "op") (or (= tv "(") (= tv "[") (= tv "{") (= tv "!"))) true)
                 (else false)))))
         (define parse-atom-postfix
           (fn ()
@@ -568,7 +568,12 @@
                     (when
                       (at-app-start?)
                       (let
-                        ((arg (parse-atom-postfix)))
+                        ((arg
+                          (cond
+                            ((at-op? "!")
+                              (begin (advance-tok!)
+                                     (list :deref (parse-atom-postfix))))
+                            (else (parse-atom-postfix)))))
                         (begin (set! head (list :app head arg)) (loop))))))
                 (loop)
                 head))))
diff --git a/lib/ocaml/runtime.sx b/lib/ocaml/runtime.sx
index 97d1835a..142eb3f2 100644
--- a/lib/ocaml/runtime.sx
+++ b/lib/ocaml/runtime.sx
@@ -366,6 +366,16 @@
      let printf fmt = print_string fmt
    end ;;
 
+   module Buffer = struct
+     let create _ = ref []
+     let add_string b s = b := s :: !b
+     let add_char b c = b := c :: !b
+     let contents b = String.concat \"\" (List.rev !b)
+     let length b = String.length (String.concat \"\" (List.rev !b))
+     let clear b = b := []
+     let reset = clear
+   end ;;
+
    module Sys = struct
      let os_type = \"SX\"
      let word_size = 64
diff --git a/lib/ocaml/test.sh b/lib/ocaml/test.sh
index 0a0e75ba..23206812 100755
--- a/lib/ocaml/test.sh
+++ b/lib/ocaml/test.sh
@@ -1044,6 +1044,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3403)
 (eval "(ocaml-run-program \"module IntOrd = struct let compare a b = compare a b end ;; module IntSet = Set.Make(IntOrd) ;; let a = IntSet.add 1 (IntSet.add 2 (IntSet.add 3 IntSet.empty)) ;; let b = IntSet.add 2 (IntSet.add 3 (IntSet.add 4 IntSet.empty)) ;; IntSet.elements (IntSet.inter a b)\")")
 
+;; ── Buffer module ──────────────────────────────────────────────
+(epoch 3500)
+(eval "(ocaml-run-program \"let b = Buffer.create 16 ;; Buffer.add_string b \\\"Hello\\\" ;; Buffer.add_string b \\\", \\\" ;; Buffer.add_string b \\\"World\\\" ;; Buffer.contents b\")")
+(epoch 3501)
+(eval "(ocaml-run-program \"let b = Buffer.create 16 ;; Buffer.add_string b \\\"abc\\\" ;; Buffer.length b\")")
+(epoch 3502)
+(eval "(ocaml-run-program \"let b = Buffer.create 16 ;; Buffer.add_string b \\\"x\\\" ;; Buffer.clear b ;; Buffer.contents b\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1652,6 +1660,11 @@ check 3401 "Map.is_empty false"           'false'
 check 3402 "Set.union"                    '(1 2 3)'
 check 3403 "Set.inter"                    '(2 3)'
 
+# ── Buffer module ──────────────────────────────────────────────
+check 3500 "Buffer concat 'Hello, World'" '"Hello, World"'
+check 3501 "Buffer.length 3"              '3'
+check 3502 "Buffer.clear empties"         '""'
+
 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 b44bcece..c494ee9c 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -264,6 +264,9 @@ SX CEK evaluator (both JS and OCaml hosts)
 - [~] `Hashtbl`: `create`, `add`, `find`, `find_opt`, `replace`, `mem`,
       `length`. Backed by a one-element list cell holding a SX dict;
       keys coerced to strings via `str` for polymorphic-key support.
+- [~] `Buffer`: `create`, `add_string`, `add_char`, `contents`, `length`,
+      `clear`, `reset`. Backed by a ref holding a list of strings; reverse +
+      `String.concat` on `contents`. Mostly-OCaml impl.
 - [~] `Sys`: `os_type` (`"SX"`), `word_size`, `max_array_length`,
       `max_string_length`, `executable_name`, `big_endian`, `unix`,
       `win32`, `cygwin`. Constants only; `argv`/`getenv_opt`/`command`
@@ -386,6 +389,11 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 1+6 — Buffer module + parser fix for `f !x` (+3
+  tests, 425 total). Parser: at-app-start? and parse-app's loop now
+  recognise `!` as the prefix-deref of an application argument, so
+  `String.concat "" (List.rev !b)` parses as `(... (deref b))`. Buffer
+  uses a ref holding a string list; contents reverses and concats.
 - 2026-05-08 Phase 5.1 — word_count.ml baseline (10/10 pass). Uses
   Map.Make(StrOrd) + List.fold_left to count word frequencies; tests
   the full functor pipeline with a real OCaml idiom.