Start here. This is the direct spoken answer to practice first.
Overview
Object-oriented programming is often explained only through examples or a list of four principles. A useful interview answer should first define the paradigm, then show how it affects responsibilities and collaboration in real C# code.
Object-oriented programming, or OOP, is a programming paradigm that models software as collaborating objects, where each object combines state with behavior and is defined by a type such as a class. In C#, an Order object can own its items and expose operations such as AddItem and Confirm instead of letting any caller change its fields freely. Encapsulation, abstraction, and polymorphism help objects protect rules and collaborate through clear contracts; inheritance is one available reuse mechanism, not a requirement. OOP is a way to organize code, not a guarantee that the resulting design is maintainable.