How it happened is well chronicled in the history paper I was asked to write by the ACM in 1992-3 "The Early History of Smalltalk" (i.e. why speculate when most high res answers are available online?). Ivan Sutherland's Sketchpad was the big first hit for me in 1966, and I saw the first Simula a week later.
Some of the confusion here is explained in the above link I wrote explaining that what is called "O-O" is nothing like I had in mind. The term was "colonized" because what we had been doing at Parc was powerful and we called it "object-oriented". But the "low-pass filter of O-O" was that Bjarne Strousrup decided to "do to C what had been done to Algol to make Simula". This was a perfectly reasonable idea. We were quite a bit more radical at Parc because we needed enormous amounts of expressability to invent personal computing (and we could and did design and build hardware that was matched to the radical ideas).
An interesting historical note is that the two inventors of Simula had completely different views of what they were doing and how it should be used for programming. Dahl was brilliant and conservative, and later wrote papers about using class definitions to make Abstract Data Types (and that is how a lot of so-called OOP programming is done today). Nygaard on the other hand was quite a wonderful wild man and visionary -- beyond brilliant -- and was into the abstract "simulate the meaningful structures" idea. Dahl was trying to fix the past and Nygaard was trying to invent the future.
During the 1978 HOPL conference time, we were visited at Parc at two different times, first by Dahl and Tony Hoare, to whom I showed and tried to explain Smalltalk: they didn't get it, and this was very frustrating. A few days later Nygaard showed up, and it was completely different: he got everything, and was finishing my sentences after 5 minutes!
Back to the origins in '66: besides the biological metaphors I had under my belt, I was also interested in the "virtual machines" ideas that were being used to do multi-processes and time-sharing in a safe way via hardware memory protection schemes (none of which I had had any involvement with inventing). One of the things that popped into my head while contemplating Sketchpad and Simula was that it would be just incredibly good -- and a huge improvement in systems designs -- to be able to use protected processes as the sole building blocks communicating only by messages that were not commands, but only "suggestions".
I think I'd call this not an invention, but a "realization" -- that the recursive machine idea was already around, and what it lacked was scalability downwards, so that all entities could be "protected virtual machines". A lot of what I now have to call "real OOP" turned out to be some years of software engineering in order to create a practical systems material that could scale in all directions.
Still seems like something worth being proud of. It just got sideswiped by the personal computer industry (a lot of things did).