OCaml doesn’t have unsigned integers (see Unsigned 32-bit and 64-bit integers by yallop · Pull Request #1201 · ocaml/ocaml · GitHub), and the convention is to use e.g. int64 for both signed and unsigned 64-bit ints:
I tend to think of fixed-width integer types (int32, int64) as bit vectors that can be used as signed integers, unsigned integers, or tuples of bits, depending on the operations applied to them.
This does mean you have to be very careful about how you use them. e.g. use an unsigned format when printing them, avoid polymorphic compare, etc.