What are the core concepts of OOP?

Reading this post makes me confused: https://news.ycombinator.com/item?id=12495117

And definitely I have heard a lot about OOP, Java, message systems, Smalltalk. Mostly I program with JavaScript and ClojureScript and these things are kind of fuzzy to me, even though I tried to learn many languages before(syntax and basic usage only) for fun.

But what is the core of OOP, that decides if one languages is or is not OOP?