I think interfacing functional code from object-oriented code is usually pretty easy. You just wrap the functions to an object instance.

However, if your 'outer layer' is functional, and try to integrate OO pieces inside it, it can be troublesome to to wrap your OO code in such a way that it's natural to use in a functional context.

OO paradigm is in some sense more restricting, in that it limits the client of the code to using OO-like patterns.

Does this make any sense to you?