This is so much nonsense. Contracts and interfaces have little to do with OOP. If you don't like inheritance, then talk about OOP. It's fine. Many people, including myself, don't like inheritance. On the other hand, if it's interfaces and contracts that have you bothered, parse-dont-validate to your heart's content. If the domain allows it. Just don't drag unrelated concepts into this particular discussion.
OOP is about messages according to the inventor - messages are about the interface. People get confused with objects which languages without messages don't have (you can get them, but they are not first class in the language)
Even plain objects though, the point isn't the inheritance! The point is to put an interface on the data. Inheritance is sometimes useful because, but there is a reason we keep screaming "prefer composition to inheritance" (even though few listen)
Just noticed that I wasn't clear - there are languages that have OBJECTS that do not do message passing. Message passing seems to be dynamic (duck type) function calls, which is debatable if it is worth it over static typing. You can get message passing in any language, but it is a lot more work and of questionable benefit vs static interfaces (there are pros and cons)
I agree with you, that inheritance isn't all of OOP, but I think it is one of THE unique selling point of OOP. All the other things like abstraction, interfaces, encapsulation, polymorphy, etc. all exist(ed) before and without OOP.
