It doesn't, it colors some keywords, it tries to follow symbols, but doesn't know the difference between 'let rec' and 'let', so it's tricky.
edit: it's a nightmare of dependency hell to install merlin, I'm giving up for now.
OCaml has a relatively immature ecosystem, but merlin is a great tool.
https://gist.github.com/nraynaud/deeacbb67c88646fd6754edba6e...
Sorry, I'm a bit abrasive, but I spend a lot of time with crappy tooling of all kind, sometimes I'm envious of monoproject people (and then I remember that actually I don't do well in monotony).