That you can do functional or OOP in C does not make C either kind of language, it just means that C is flexible enough that you can make the computer do things the way you want it to, no matter what that means, and other languages purposefully prevent you from doing what you might want to do.

C++ is object oriented not because it has compile time support for polymorphism or any of that other bad programming practice, but because classes have code sections that live with them, whether on the stack or in the heap, that can operate only on memory belonging to that instance of the class.

Object oriented programming is a coding style and choice. Some languages make it a first class part of the language design. It is purposefully not part of C.

However you can do OOP like things in C: a popular paradigm is to pass around pointers to structs that (should) live in the heap, and to have a number of functions which work on these structs. This is very similar in practice and mental modelling to OOP as users of C++ might know it, but is distinct in that no code ever lives in the stack or heap, and no code is restricted from operating on any of the program memory.