Having used fork/join to parallelize an OCaml genetic algorithm, I can say from experience it is neither safe nor practical.
Edit: Intentionally only supporting single-threaded processes is a perfectly fine design decision, however I haven't seen this argument made for OCaml, rather I've seen "multi-threading our GC would be hard" as the justification for the single-thread limitation. Admittedly it was 3-4 years ago when I last had to deal with this.
There are now several experimental multithreaded OCaml runtimes floating around, which also have the key property of not slowing down the single-threaded case. I don't expect multicore to remain a limitation for OCaml in 2014.
Edit: Intentionally only supporting single-threaded processes is a perfectly fine design decision, however I haven't seen this argument made for OCaml, rather I've seen "multi-threading our GC would be hard" as the justification for the single-thread limitation. Admittedly it was 3-4 years ago when I last had to deal with this.