It's even harder to define (which might be why OOP is so popular --- it's easy to get dogmatic about "OOP is X", and the cargo-culters will pick it up and spread it like religion), but I'd say "be pragmatic and let it come naturally". Systems tend to naturally form OO-ish things even if they're not written in an OO language; the Linux kernel is one example. I think of OO as just another layer of organisation beyond functions and structures, arising from the desire to group related data and code together.