They are radically different languages, even if they're from the same family. Scala is about as similar to OCaml as Rust is to OCaml. Rust and Scala are a world apart from OCaml and Standard ML in terms of language complexity.

And then F# literally has an OCaml compat mode because they are so similar (F# has a bunch of convenient syntax extensions if you don't mind not using the compat mode).

I don't know OCaml, so I can't give you a great answer, but I know that OCaml has a different type system that has some features that Scala 2 lacks, such as union types.

OCaml doesn't have union types. It has what it calls variants, and what is generally known as Algebraic Data Types. So does Scala 2.