ReasonML is just an alternative syntax for OCaml. Often people use "ReasonML" shorthand for "ReasonML, converted to OCaml, compiled to Javascript with BuckleScript" and "OCaml" for "OCaml compiled natively" but it's perfectly possible (and actually really easy) to use OCaml with BuckleScript and Reason natively.
Also note that despite the "O" in the name, OOP in OCaml is rare (for instance, there isn't any in the code in the article).
Also note that despite the "O" in the name, OOP in OCaml is rare (for instance, there isn't any in the code in the article).