If you'll forgive the shameless plug, I talked a bit about ReasonML here, hopefully it helps clear things up https://youtu.be/QWfHrbSqnB0?t=29m34s

Basically, just a new syntax and a blessed-stack approach that really, really emphasizes developer experience. Which is to say, ReasonML is merely a cosmetic and DX change, it remains compatible and is not a fork of OCaml at all.