Shameless plug: there's also a newish O'Reilly book, Real World OCaml http://realworldocaml.org, which I think is a big help in learning the language.
Oh, and there's OCaml Labs, a new lab at Cambridge University that's dedicated to improving the language.
And of course the compiler is constantly making progress. The upcoming 4.02 release is a pretty fun one, which I documented here: https://blogs.janestreet.com/ocaml-4-02-everything-else/ And before that, changes like GADTs and first-class modules landed, which have been quite useful extensions.
Really, it's a very active and fun community these days. The language is getting better quite quickly, but mostly in conservative and tasteful ways. The people in charge of the core language have been doing a great job, and the community infrastructure (things like OPAM) have been making big strides as well.
- OPAM was already mentioned.
- The OCaml to JS compiler has matured and gained a lot of use. Interestingly, I suspect that it can also be used to achieve different types of parallelism than the types that the stock OCaml compilers provide. https://github.com/ocsigen/js_of_ocaml
- Merlin is also a fantastic autocompletion plugin for Vim/Emacs. https://github.com/the-lambda-church/merlin
VimBox (shameless plug) uses Merlin and is configured to provide "intellisense" style completion (as you type) like Visual Studio. https://github.com/jordwalke/VimBox
- utop is perhaps the best top level REPL that I've ever seen. https://github.com/diml/utop (edit: Regarding utop: Normally I dread using a REPL for a statically typed language, but utop actually makes the top level fun and interactive. I actually look forward to using it!)
I've tried it a few times through the years. Most recently I grabbed a kindle version of "real world ocaml" a couple months ago, and started to work through it. The fact there's a recent book in English made it easier to get into than the previous times I've looked at it, and a good sign of progress, IMO.
Unfortunately, after a couple weeks a new version of ocaml came out, and some packaged from opam were updated, and something about it broke everything for me and stopped me in my tracks. I'm pretty sure it's a problem waiting for changes to trickle through to osx, but I don't really know. I spun up a linux vm, and everything worked great, so it seems osx specific.