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.
module Nativeint: sig .. end
   This module provides operations on the type nativeint of
   signed 32-bit integers (on 32-bit platforms) or
   signed 64-bit integers (on 64-bit platforms).
   This integer type has exactly the same width as that of a
   pointer type in the C compiler.  All arithmetic operations over
   nativeint are taken modulo 232 or 264 depending
   on the word size of the architecture.
   Performance notice: values of type nativeint occupy more memory
   space than values of type int, and arithmetic operations on
   nativeint are generally slower than those on int.  Use nativeint
   only when the application requires the extra bit of precision
   over the int type.
val zero : nativeint
val one : nativeint
val minus_one : nativeint
val neg : nativeint -> nativeint
val add : nativeint -> nativeint -> nativeint
val sub : nativeint -> nativeint -> nativeint
val mul : nativeint -> nativeint -> nativeint
val div : nativeint -> nativeint -> nativeint
Division_by_zero if the second
   argument is zero.  This division rounds the real quotient of
   its arguments towards zero, as specified for Pervasives.(/).val rem : nativeint -> nativeint -> nativeint
y is not zero, the result
   of Nativeint.rem x y satisfies the following properties:
   Nativeint.zero <= Nativeint.rem x y < Nativeint.abs y and
   x = Nativeint.add (Nativeint.mul (Nativeint.div x y) y)
                      (Nativeint.rem x y).
   If y = 0, Nativeint.rem x y raises Division_by_zero.val succ : nativeint -> nativeint
Nativeint.succ x is Nativeint.add x Nativeint.one.val pred : nativeint -> nativeint
Nativeint.pred x is Nativeint.sub x Nativeint.one.val abs : nativeint -> nativeint
val size : int
32
   on a 32-bit platform and to 64 on a 64-bit platform.val max_int : nativeint
val min_int : nativeint
val logand : nativeint -> nativeint -> nativeint
val logor : nativeint -> nativeint -> nativeint
val logxor : nativeint -> nativeint -> nativeint
val lognot : nativeint -> nativeint
val shift_left : nativeint -> int -> nativeint
Nativeint.shift_left x y shifts x to the left by y bits.
   The result is unspecified if y < 0 or y >= bitsize,
   where bitsize is 32 on a 32-bit platform and
   64 on a 64-bit platform.val shift_right : nativeint -> int -> nativeint
Nativeint.shift_right x y shifts x to the right by y bits.
   This is an arithmetic shift: the sign bit of x is replicated
   and inserted in the vacated bits.
   The result is unspecified if y < 0 or y >= bitsize.val shift_right_logical : nativeint -> int -> nativeint
Nativeint.shift_right_logical x y shifts x to the right
   by y bits.
   This is a logical shift: zeroes are inserted in the vacated bits
   regardless of the sign of x.
   The result is unspecified if y < 0 or y >= bitsize.val of_int : int -> nativeint
int) to a native integer
   (type nativeint).val to_int : nativeint -> int
nativeint) to an
   integer (type int).  The high-order bit is lost during
   the conversion.val of_float : float -> nativeint
Nativeint.min_int, Nativeint.max_int].val to_float : nativeint -> float
val of_int32 : int32 -> nativeint
int32)
   to a native integer.val to_int32 : nativeint -> int32
int32).  On 64-bit platforms,
   the 64-bit native integer is taken modulo 232,
   i.e. the top 32 bits are lost.  On 32-bit platforms,
   the conversion is exact.val of_string : string -> nativeint
0x, 0o or 0b
   respectively.
   Raise Failure "int_of_string" if the given string is not
   a valid representation of an integer, or if the integer represented
   exceeds the range of integers representable in type nativeint.val to_string : nativeint -> string
type t = nativeint 
val compare : t -> t -> int
Pervasives.compare.  Along with the type t, this function compare
    allows the module Nativeint to be passed as argument to the functors
    Set.Make and Map.Make.val format : string -> nativeint -> string
Nativeint.format fmt n return the string representation of the
   native integer n in the format specified by fmt.
   fmt is a Printf-style format consisting of exactly
   one %d, %i, %u, %x, %X or %o conversion specification.
   This function is deprecated; use Printf.sprintf with a %nx format
   instead.