[nickpsecurity]

It started in CompSci sort of in parallel with Bob Barton's B5000 designed for ALGOL, the work of Dijkstra, McCarthy's LISP, Hamilton's USL, and so on. They each came up with abstract ways to specify or implement programs for greater correctness, readability, safety, and composition. Using provers, compilers, or manual work, these would be converted into specific code at lower levels or final, machine level in ways intended to preserve high-level properties. Dijkstra went furthest in THE where the did hierarchical layers that were loop free. In parallel, there was a group trying to figure out how to compile Monte Carlo simulations on their computers in a way that was easy to specify. Their Simula was first, OOP language. SimScript, a RAND language, was a discrete, event simulation tool that came out with many similarities to emerging Simula and OOP. The main programmer had already been exposed to ALGOL, loved the heck out of it, and took some inspiration from SimScript.

Hierarchical layering, careful interfaces, dynamic programming, functional composition, refinement, event-based simulation... all showing up before Simula was published in 1967. So, it anywhere from depended on to came after many things with properties key to OOP's effectiveness. It certainly got a powerful technique for structuring programs started but didn't happen in isolation or even necessarily ahead in many ways. It surprised me that needs of Monte Carlo apps is what led to OOP but not that ALGOL60 was involved.

http://phobos.ramapo.edu/~ldant/oop/simula_history.pdf

https://www.rand.org/content/dam/rand/pubs/research_memorand...