Okay, thanks! But doesn't this still mean you have to compile ocaml (with flambda) yourself?

If the derivation has already been written (as GP implies), then compiling something yourself with Nix is essentially just a longer installation process. No downloading tarballs, no keeping track of things in /usr/local, no stow or manually adding things to your path. Nix is essentially a source distribution that pulls common binaries from caches.