It’s all about the data model and the architecture.

I think people focus a lot on inheritance but the core idea of OO is more the grouping of values and functions. Conceptually, you think about how methods transforms the data you are manipulating and that’s a useful way to think about programs.

This complexity doesn’t really disappear when you leave OO language actually. The way most complex Ocaml programs are structured with modules grouping one main type and the functions working on it is in a lot of way inspired by OO.