OCaml isn't actually a pure functional language: it has side-effects and mutation. In fact, you can code that looks an awful lot like C if you want, except with automatic type inference. But then you can also lift up your level of abstraction in the same codebase to use a purely functional idiom where it's appropriate, or use an object-oriented style for something else. All of those styles are supported as consistent, first-class features in the language.

There are several event-driven programming libraries in OCaml, and two that I like that use monadic abstractions (to hide the messy control flow that makes node.js so painful) are Async (https://ocaml.janestreet.com/?q=node%2F100) and Lwt (http://ocsigen.org).

You have good low-level memory layout in OCaml, either via an FFI or the Ancient module to import in non-GCed values. You can read about the OCaml heap and GC at this blog series by Richard Jones. http://rwmj.wordpress.com/2009/08/04/ocaml-internals/ (and yes, the memory representation is a straightforward mapping from the type declaration; very little magic happens in the compiler)

OCaml is used all over the Xen Cloud Platform (http://github.com/xen-org/xen-api), and I'm running an ongoing microkernel research project called Mirage (http://www.openmirage.org) which has a full network stack written in pure OCaml (and is also competitive performance-wise).