Current OCaml needn't worry about data races since no state is shared. Unfortunately, current OCaml also only exploits a single core, but it's so insanely fast that usually doesn't matter.

Upcoming OCaml multicore solves the shared state and data races problem with algebraic effects[0]. It is working, it's just not yet been released in mainline OCaml. Supposedly, that will happen this year, but we'll see.

0. http://kcsrk.info/ocaml/multicore/2015/05/20/effects-multico...