Object-oriented programming (OOP) is a way of writing computer programs using objects that represent real-world things. These objects can have data (attributes) and methods (functions) that operate on the data. OOP helps organize code neatly and makes it easier to reuse and modify. Polymorphism in Java allows objects of different types to be treated as instances of the same class, enabling methods to behave differently based on their actual types. This is achieved through method overloading (compile-time polymorphism) and method overriding (runtime polymorphism).