I would put OOP's solid theoretical foundations at the Liskov Substitution Principle invented in ~1988. This seems relatively late compared to OO's popularity.

I would put OOP's solid theoretical foundations at the Liskov Substitution Principle invented in ~1988. This seems relatively late compared to OO's popularity. It was less than ~10 years between "structured programming" and the structured programming theorem (and lots of useful formal steps on the way), vs. 20+ for OOP and LSP (and at least at today's distance, LSP along with Meyer's contemporary substitution principles seems more like a sea change).