In my opinion, one way of being an efficient ocaml developer is to keep it simple.
Ocaml is a surprisingly good friction-less programming languages that allow you to prototype very fast. At the same time it has very advanced features that let you express some problems in very abstract/complex way.
This is a trade-off in any languages, but i think ocaml has a un-usually wide spectrum to very simple code to functor-heavy with GADT and object phantom-type code only few people can read/write.
On top of that, there are lots of brights programmers and researcher in ocaml community who write clean complex code that might make it looks like it’s easy.
My point is, sometimes complex code/feature is a good, sometimes its fun, but if you want to be efficient, then keep it as simple as possible.