A lot of the Linux kernel these days is basically OO, at least the parts I look at. It's not in an OO language, but the concepts and patterns are there: an opaque void * "object" that's only accessed via a set of associated "methods", which all have manually name-mangled names like foo_method1 and foo_method2. These "objects" are created via a "constructor" foo_create and destroyed via a manually called "destructor" foo_free, etc. There's even a little bit of manually implemented inheritance and virtual-function stuff going on with function-pointers-in-a-struct type patterns.