Simula can't be praised too highly, and it was a huge influence. But if you take the care to check, you will find out that what I termed "Object Oriented" was quite different from Simula in (what I thought were) important ways. And if you are looking for the earliest inventions of ideas like these, there are several that predate Simula (take a look at that HOPL II chapter mentioned above)

I certainly don't want to underemphasise the huge contribution of Smalltalk.

Nevertheless, consider a couple of modern widely-used languages. C++ in terms of object-orientation, is based on Simula, and Java's object model is entirely based on Simula.

I argue that it was Nygaard and Dahl who first introduced those language's key concepts (objects, classes, sub-classing and virtual functions) in a coherent and programmable form. This is why Nygaard's Turing Award page states "Kristen Nygaard is internationally acknowledged as the co-inventor with Ole-Johan Dahl of object-oriented programming and the programming language SIMULA", and why I stated in my parent response that you named the term others invented, notwithstanding that today's common use isn't necessarily consistent with your coining of the term.