How does OCP relate to interfaces and abstract classes?

Short answer: Interfaces and abstract classes provide a contract that other classes can implement or inherit.

Explain a bit more

They enable polymorphism, which allows behavior to be extended without altering existing code. By programming to abstractions (not concrete implementations), you can easily introduce new behavior (via new implementations) while keeping the core logic unchanged. ✅ OCP encourages extending via new subclasses or interface implementations, not modifying existing ones. Liskov Substitution Principle (LSP)

Real-world example (ShopNest)

Open/Closed in ShopNest: add a new payment method by adding a class, not by editing a giant switch in CheckoutService.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.