Additionally, the ReasonML team aims to improve the OCaml ecosystem:
Better tooling (testing, documentation, editor support, etc.).
Better standard library (there is a fair amount of competition in this space in OCaml, without a clear winner). Here, the goal is also to use camel-cased names (StringUtilities, createResource). OCaml uses snake-casing for lowercase names (create_resource) and camel-casing for uppercase names (StringUtilities).
Benefits of OCaml:
It is an established language (created in 1996) that has proven itself in many projects. Facebook itself is using it in several projects (e.g. Flow).
Its core is a functional programming language (with a full-featured type system). But it also supports object-orientation and mutable state.