I had been programming for a long time before I learned OOP. After some years playing with it, I came to the conclusion there's not much I can't do about as well using simple functions and structs. The key is a well thought out and organized codebase. Always felt polymorphism in particular seemed more trouble than it was worth.

I still use modern languages on a regular basis, but when I drop back to more basic languages there are only a few ergonomics that I truly miss (eg. generics).