Below is the API for the OCaml standard library. It's directly copied over from the OCaml Manual, formatted to the Reason syntax and styled accordingly. The API docs are work-in-progress; we'll be polishing these gradually!
If you're targeting JavaScript, the API docs for BuckleScript includes all of below, plus JS-specific APIs.
sig external raise : exn -> 'a = "%raise" external raise_notrace : exn -> 'a = "%raise_notrace" val invalid_arg : string -> 'a val failwith : string -> 'a exception Exit external ( = ) : 'a -> 'a -> bool = "%equal" external ( <> ) : 'a -> 'a -> bool = "%notequal" external ( < ) : 'a -> 'a -> bool = "%lessthan" external ( > ) : 'a -> 'a -> bool = "%greaterthan" external ( <= ) : 'a -> 'a -> bool = "%lessequal" external ( >= ) : 'a -> 'a -> bool = "%greaterequal" external compare : 'a -> 'a -> int = "%compare" val min : 'a -> 'a -> 'a val max : 'a -> 'a -> 'a external ( == ) : 'a -> 'a -> bool = "%eq" external ( != ) : 'a -> 'a -> bool = "%noteq" external not : bool -> bool = "%boolnot" external ( && ) : bool -> bool -> bool = "%sequand" external ( & ) : bool -> bool -> bool = "%sequand" external ( || ) : bool -> bool -> bool = "%sequor" external ( or ) : bool -> bool -> bool = "%sequor" external __LOC__ : string = "%loc_LOC" external __FILE__ : string = "%loc_FILE" external __LINE__ : int = "%loc_LINE" external __MODULE__ : string = "%loc_MODULE" external __POS__ : string * int * int * int = "%loc_POS" external __LOC_OF__ : 'a -> string * 'a = "%loc_LOC" external __LINE_OF__ : 'a -> int * 'a = "%loc_LINE" external __POS_OF__ : 'a -> (string * int * int * int) * 'a = "%loc_POS" external ( |> ) : 'a -> ('a -> 'b) -> 'b = "%revapply" external ( @@ ) : ('a -> 'b) -> 'a -> 'b = "%apply" external ( ~- ) : int -> int = "%negint" external ( ~+ ) : int -> int = "%identity" external succ : int -> int = "%succint" external pred : int -> int = "%predint" external ( + ) : int -> int -> int = "%addint" external ( - ) : int -> int -> int = "%subint" external ( * ) : int -> int -> int = "%mulint" external ( / ) : int -> int -> int = "%divint" external ( mod ) : int -> int -> int = "%modint" val abs : int -> int val max_int : int val min_int : int external ( land ) : int -> int -> int = "%andint" external ( lor ) : int -> int -> int = "%orint" external ( lxor ) : int -> int -> int = "%xorint" val lnot : int -> int external ( lsl ) : int -> int -> int = "%lslint" external ( lsr ) : int -> int -> int = "%lsrint" external ( asr ) : int -> int -> int = "%asrint" external ( ~-. ) : float -> float = "%negfloat" external ( ~+. ) : float -> float = "%identity" external ( +. ) : float -> float -> float = "%addfloat" external ( -. ) : float -> float -> float = "%subfloat" external ( *. ) : float -> float -> float = "%mulfloat" external ( /. ) : float -> float -> float = "%divfloat" external ( ** ) : float -> float -> float = "caml_power_float" "pow" "float" external sqrt : float -> float = "caml_sqrt_float" "sqrt" "float" external exp : float -> float = "caml_exp_float" "exp" "float" external log : float -> float = "caml_log_float" "log" "float" external log10 : float -> float = "caml_log10_float" "log10" "float" external expm1 : float -> float = "caml_expm1_float" "caml_expm1" "float" external log1p : float -> float = "caml_log1p_float" "caml_log1p" "float" external cos : float -> float = "caml_cos_float" "cos" "float" external sin : float -> float = "caml_sin_float" "sin" "float" external tan : float -> float = "caml_tan_float" "tan" "float" external acos : float -> float = "caml_acos_float" "acos" "float" external asin : float -> float = "caml_asin_float" "asin" "float" external atan : float -> float = "caml_atan_float" "atan" "float" external atan2 : float -> float -> float = "caml_atan2_float" "atan2" "float" external hypot : float -> float -> float = "caml_hypot_float" "caml_hypot" "float" external cosh : float -> float = "caml_cosh_float" "cosh" "float" external sinh : float -> float = "caml_sinh_float" "sinh" "float" external tanh : float -> float = "caml_tanh_float" "tanh" "float" external ceil : float -> float = "caml_ceil_float" "ceil" "float" external floor : float -> float = "caml_floor_float" "floor" "float" external abs_float : float -> float = "%absfloat" external copysign : float -> float -> float = "caml_copysign_float" "caml_copysign" "float" external mod_float : float -> float -> float = "caml_fmod_float" "fmod" "float" external frexp : float -> float * int = "caml_frexp_float" external ldexp : float -> int -> float = "caml_ldexp_float" external modf : float -> float * float = "caml_modf_float" external float : int -> float = "%floatofint" external float_of_int : int -> float = "%floatofint" external truncate : float -> int = "%intoffloat" external int_of_float : float -> int = "%intoffloat" val infinity : float val neg_infinity : float val nan : float val max_float : float val min_float : float val epsilon_float : float type fpclass = FP_normal | FP_subnormal | FP_zero | FP_infinite | FP_nan external classify_float : float -> Pervasives.fpclass = "caml_classify_float" val ( ^ ) : string -> string -> string external int_of_char : char -> int = "%identity" val char_of_int : int -> char external ignore : 'a -> unit = "%ignore" val string_of_bool : bool -> string val bool_of_string : string -> bool val string_of_int : int -> string external int_of_string : string -> int = "caml_int_of_string" val string_of_float : float -> string external float_of_string : string -> float = "caml_float_of_string" external fst : 'a * 'b -> 'a = "%field0" external snd : 'a * 'b -> 'b = "%field1" val ( @ ) : 'a list -> 'a list -> 'a list type in_channel type out_channel val stdin : Pervasives.in_channel val stdout : Pervasives.out_channel val stderr : Pervasives.out_channel val print_char : char -> unit val print_string : string -> unit val print_bytes : bytes -> unit val print_int : int -> unit val print_float : float -> unit val print_endline : string -> unit val print_newline : unit -> unit val prerr_char : char -> unit val prerr_string : string -> unit val prerr_bytes : bytes -> unit val prerr_int : int -> unit val prerr_float : float -> unit val prerr_endline : string -> unit val prerr_newline : unit -> unit val read_line : unit -> string val read_int : unit -> int val read_float : unit -> float type open_flag = Open_rdonly | Open_wronly | Open_append | Open_creat | Open_trunc | Open_excl | Open_binary | Open_text | Open_nonblock val open_out : string -> Pervasives.out_channel val open_out_bin : string -> Pervasives.out_channel val open_out_gen : Pervasives.open_flag list -> int -> string -> Pervasives.out_channel val flush : Pervasives.out_channel -> unit val flush_all : unit -> unit val output_char : Pervasives.out_channel -> char -> unit val output_string : Pervasives.out_channel -> string -> unit val output_bytes : Pervasives.out_channel -> bytes -> unit val output : Pervasives.out_channel -> bytes -> int -> int -> unit val output_substring : Pervasives.out_channel -> string -> int -> int -> unit val output_byte : Pervasives.out_channel -> int -> unit val output_binary_int : Pervasives.out_channel -> int -> unit val output_value : Pervasives.out_channel -> 'a -> unit val seek_out : Pervasives.out_channel -> int -> unit val pos_out : Pervasives.out_channel -> int val out_channel_length : Pervasives.out_channel -> int val close_out : Pervasives.out_channel -> unit val close_out_noerr : Pervasives.out_channel -> unit val set_binary_mode_out : Pervasives.out_channel -> bool -> unit val open_in : string -> Pervasives.in_channel val open_in_bin : string -> Pervasives.in_channel val open_in_gen : Pervasives.open_flag list -> int -> string -> Pervasives.in_channel val input_char : Pervasives.in_channel -> char val input_line : Pervasives.in_channel -> string val input : Pervasives.in_channel -> bytes -> int -> int -> int val really_input : Pervasives.in_channel -> bytes -> int -> int -> unit val really_input_string : Pervasives.in_channel -> int -> string val input_byte : Pervasives.in_channel -> int val input_binary_int : Pervasives.in_channel -> int val input_value : Pervasives.in_channel -> 'a val seek_in : Pervasives.in_channel -> int -> unit val pos_in : Pervasives.in_channel -> int val in_channel_length : Pervasives.in_channel -> int val close_in : Pervasives.in_channel -> unit val close_in_noerr : Pervasives.in_channel -> unit val set_binary_mode_in : Pervasives.in_channel -> bool -> unit module LargeFile : sig val seek_out : Pervasives.out_channel -> int64 -> unit val pos_out : Pervasives.out_channel -> int64 val out_channel_length : Pervasives.out_channel -> int64 val seek_in : Pervasives.in_channel -> int64 -> unit val pos_in : Pervasives.in_channel -> int64 val in_channel_length : Pervasives.in_channel -> int64 end type 'a ref = { mutable contents : 'a; } external ref : 'a -> 'a Pervasives.ref = "%makemutable" external ( ! ) : 'a Pervasives.ref -> 'a = "%field0" external ( := ) : 'a Pervasives.ref -> 'a -> unit = "%setfield0" external incr : int Pervasives.ref -> unit = "%incr" external decr : int Pervasives.ref -> unit = "%decr" type ('a, 'b, 'c, 'd, 'e, 'f) format6 = ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.format6 type ('a, 'b, 'c, 'd) format4 = ('a, 'b, 'c, 'c, 'c, 'd) Pervasives.format6 type ('a, 'b, 'c) format = ('a, 'b, 'c, 'c) Pervasives.format4 val string_of_format : ('a, 'b, 'c, 'd, 'e, 'f) Pervasives.format6 -> string external format_of_string : ('a, 'b, 'c, 'd, 'e, 'f) Pervasives.format6 -> ('a, 'b, 'c, 'd, 'e, 'f) Pervasives.format6 = "%identity" val ( ^^ ) : ('a, 'b, 'c, 'd, 'e, 'f) Pervasives.format6 -> ('f, 'b, 'c, 'e, 'g, 'h) Pervasives.format6 -> ('a, 'b, 'c, 'd, 'g, 'h) Pervasives.format6 val exit : int -> 'a val at_exit : (unit -> unit) -> unit val valid_float_lexem : string -> string val unsafe_really_input : Pervasives.in_channel -> bytes -> int -> int -> unit val do_at_exit : unit -> unit end