A useful tool for writing OCaml in Vim is Merlin [0].
Facebook uses OCaml as well [1] [2] [3]
A great tutorial is [4]
[0] https://github.com/ocaml/merlin
[1] https://github.com/facebook/reason
[2] https://github.com/facebook/flow/
[3] https://github.com/facebook/infer
[4] http://realworldocaml.org/