Even though it's not OO that doesn't stop people adding their own hacked OO to C. I think the Quake source code had a lot of this, structs with function pointers. OO is such a natural fit for games in particular it finds a way to exist.

The whole GObject ecosystem is hacky object oriented C, to provide another suite of examples.

I personally found hacked OO ala Perl to be the first time I really understood it, even after years of class diagrams at university. The abstraction got in the way. I needed the nuts and bolts.

Yeah, I delved into it when I was working with C, but by that time I had no interest in reproducing inheritance but only interfaces.