How does OCP promote extensibility?

OCP promotes extensibility by encouraging a design where new behavior is added through new code, not by modifying existing code. This is achieved by identifying the variation points in a system and abstracting them behind interfaces or abstract classes. For example, a logging system that supports FileLogger and needs to add DatabaseLogger follows OCP if both implement a Logger interface — the calling code doesn't change. Systems designed with OCP in mind tend to use plugins, strategies, and hooks to allow extension points. The payoff is that the risk of regression is dramatically reduced: existing, tested code is never touched when new behavior is added.