How to make library installed from OPAM available to OCaml?

I followed this tutorial on OCaml FFI and installed Ctypes through OPAM:

opam install ctypes

However, OCaml does not find the module:

open Ctypes
(* ... *)

I receive the error:

Unbound module Ctypes

It looks like I need to let OCaml know where my Ctypes installation is? Do I need to update some path variable to let OCaml look for my libraries installed through OPAM?

This is Ubuntu 15.04, OCaml 4.01.0, OPAM 1.2.0.