Object-Oriented Design (OOD) is a programming approach that organizes software around data, or objects, rather than functions and logic. In OOD, an object is a self-contained unit that combines both data and the methods that operate on that data. This design paradigm promotes code reusability and modularity, making it easier to manage and scale complex software systems. Key concepts in OOD include classes, objects, inheritance, and polymorphism. A class serves as a blueprint for creating objects, while inheritance allows new classes to adopt properties of existing ones. Polymorphism enables objects to be treated as instances of their parent class, enhancing flexibility in code.