To apply loose coupling and testability in OOD, you need to follow some best practices and design patterns that support these principles. Utilize interfaces and abstract classes to define contracts and behaviors, and use subclasses and implementations to provide specific details and variations. Additionally, employ the SOLID principles to design cohesive, modular, and extensible classes that follow the single responsibility, open-closed, Liskov substitution, interface segregation, and dependency inversion principles. Moreover, apply the DRY principle to avoid duplication and redundancy in your code by extracting common functionality into reusable methods or classes. Furthermore, use the principle of least knowledge to limit the interaction and communication between classes, while avoiding exposing unnecessary details or data. Finally, employ design patterns such as strategy, adapter, facade, decorator, or observer to decouple classes and objects for dynamic behavior and flexibility. By following these practices and patterns, you can achieve loose coupling and testability in OOD for improved maintainability and quality of your software.