Yes, it uses ML-style type inference, but that doesn't mean you are literally unable to annotate types, and the compiler is absolutely still doing type checking. I'm sure people like Jane Street annotate everything.

In addition, perhaps a small point relative to the first, but in Ocaml, the arithmetic operators perform no type inference; there's a separate operator for float-addition versus int-addition, and so on. This somewhat limits your exposure to potential automatic type conversions.