I've heard it best as: both Ocaml and Haskell are pure functional languages. Haskell is Extra Virgin.

That said, the notion of purity is a bit BS. At some point in your program you are definitely going to invoke the world-at-large. Whether you do it through IO(), tap your nose and call it "pure" or you do it other way is all down to semantics (not in the PLT sense of the word - the PLT term to use would be "pragmatics"). The core of both Ocaml and Haskell are pure functions.

Also, Ocaml doesn't really need typeclasses. Modules. Modules are everything