OCaml - How do I convert int to string?

Use the function string_of_int (see the documentation for Pervasives, the module containing the functions that are automatically made available in the top level namespace to al OCaml programs).

Another solution is to use the module Printf, which allows you to choose the format of printing:

Printf.sprintf "%d" 42

gives you "42".

But you might prefer using an octal, hexadecimal, binary, etc representation. For instance,

Printf.sprintf "%x" 42

gives you "2a" which is the hexadecimal representation of 42.

Printf.sprintf "0x%x" 42

would give you "0x2a".

See the Printf documentation for more details.