It's impossible to get people to agree on what OOP is exactly, but almost everyone agrees that hiding/isolating mutable state (inside objects) is an essential part of it. Just google "what are objects in programming".
My point here was that all "OOP" languages (that I know) allow you, with all their "OOP feautres", to work with objects which actually don't contain any mutable state.
I'll let you be the judge whether it is still "OOP", but I know from experience that it's very practical for Software Engineering.
