Async and LWT give you lightweight concurrency mechanisms. Libraries like Async.RPC give you lightweight remote invocation, and Async_parallel gives you simple mechanisms for spinning up multiple physical processes and communicating between them.

Don't get me wrong: OTP by all accounts has richer support for this kind of stuff. I think OCaml is a better language for many purposes, but OTP is a great runtime and set of libraries whose equal is not yet found in any other language as far as I can tell.