Alan Kay may have created the term, but object-oriented programming didn't originate with Smalltalk; it originated with Simula, an ALGOL variant with classes and many of the concepts we consider to be core OOP principles today, including inheritance and virtual methods (polymorphism).
Encapsulation (private, protected, public) was not in the original Simula, but it was added to the TOPS-10 version, and was later integrated into Simula 87. From there, these core concepts have spread to the rest of the object-oriented world.
For all Alan Kay's talk of inventing OOP, it's Simula's style that has been successful. Smalltalk-style OOP has largely failed in the marketplace of ideas everywhere and every time it's been introduced, with one notable exception. Objective-C uses Smalltalk-style OOP, and it's found a good deal of success in iOS, because Apple built their whole API around it and thus shoved it down their developers' throats. Outside of the Apple ecosystem, though, Objective-C is as little used as any other implementation of OOP-by-message-passing.