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 Int64: sig .. end
   This module provides operations on the type int64 of
   signed 64-bit integers.  Unlike the built-in int type,
   the type int64 is guaranteed to be exactly 64-bit wide on all
   platforms.  All arithmetic operations over int64 are taken
   modulo 264
   Performance notice: values of type int64 occupy more memory
   space than values of type int, and arithmetic operations on
   int64 are generally slower than those on int.  Use int64
   only when the application requires exact 64-bit arithmetic.
val zero : int64
val one : int64
val minus_one : int64
val neg : int64 -> int64
val add : int64 -> int64 -> int64
val sub : int64 -> int64 -> int64
val mul : int64 -> int64 -> int64
val div : int64 -> int64 -> int64
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 : int64 -> int64 -> int64
y is not zero, the result
   of Int64.rem x y satisfies the following property:
   x = Int64.add (Int64.mul (Int64.div x y) y) (Int64.rem x y).
   If y = 0, Int64.rem x y raises Division_by_zero.val succ : int64 -> int64
Int64.succ x is Int64.add x Int64.one.val pred : int64 -> int64
Int64.pred x is Int64.sub x Int64.one.val abs : int64 -> int64
val max_int : int64
val min_int : int64
val logand : int64 -> int64 -> int64
val logor : int64 -> int64 -> int64
val logxor : int64 -> int64 -> int64
val lognot : int64 -> int64
val shift_left : int64 -> int -> int64
Int64.shift_left x y shifts x to the left by y bits.
   The result is unspecified if y < 0 or y >= 64.val shift_right : int64 -> int -> int64
Int64.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 >= 64.val shift_right_logical : int64 -> int -> int64
Int64.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 >= 64.val of_int : int -> int64
int) to a 64-bit integer
    (type int64).val to_int : int64 -> int
int64) to an
   integer (type int).  On 64-bit platforms, the 64-bit integer
   is taken modulo 263, i.e. the high-order bit is lost
   during the conversion.  On 32-bit platforms, the 64-bit integer
   is taken modulo 231, i.e. the top 33 bits are lost
   during the conversion.val of_float : float -> int64
Int64.min_int, Int64.max_int].val to_float : int64 -> float
val of_int32 : int32 -> int64
int32)
   to a 64-bit integer (type int64).val to_int32 : int64 -> int32
int64) to a
   32-bit integer (type int32). The 64-bit integer
   is taken modulo 232, i.e. the top 32 bits are lost
   during the conversion.val of_nativeint : nativeint -> int64
nativeint)
   to a 64-bit integer (type int64).val to_nativeint : int64 -> nativeint
int64) to a
   native integer.  On 32-bit platforms, the 64-bit integer
   is taken modulo 232.  On 64-bit platforms,
   the conversion is exact.val of_string : string -> int64
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 int64.val to_string : int64 -> string
val bits_of_float : float -> int64
val float_of_bits : int64 -> float
int64.type t = int64 
val compare : t -> t -> int
Pervasives.compare.  Along with the type t, this function compare
    allows the module Int64 to be passed as argument to the functors
    Set.Make and Map.Make.val format : string -> int64 -> string