OCaml's OO is IMHO not very interesting, beside the concept of "functional objects", which really ought to exist without the rest of the Java-style OO baggage.

OCaml's OO is IMHO not very interesting, beside the concept of "functional objects", which really ought to exist without the rest of the Java-style OO baggage. (Briefly: methods can easily return a copy of an object with some fields modified; and anonymous, structurally typed objects may be constructed.) Otherwise it is standard Java/C++ fare (albeit more streamlined and with better typing). On the other hand, OCaml is worth learning for the module+type system alone. Every other language could benefit from its ideas; the only language I've seen that's comparable is Coq (which bases its module system on OCaml's). (And the module and type system really work in tandem: there are advanced mechanisms for type structure hiding that aid forward compatibility.)