Functional isn't opposite of OO, because there is functional OO.

OO with immutable objects is functional. (States can still be represented by creating new objects based on old ones.)

From a functional point of view, OO is a way of organizing the types into classes which are tuples of properties optionally linked by inheritance relationships, and of dispatching methods based on class-based pattern matching over the argument lists.

Functional OO is "cleaner" than regular OO in some ways. For instance circle-ellipse issues don't arise, because you can't mutate a circle or ellipse once it is constructed.