Why does OCP reduce the number of regressions?

OCP reduces the number of regressions because it reduces the number of changes to code that already works and has been verified, and that is exactly where regressions most often occur.

In detail:

#1. Isolating new code from old

New behavior is added as new classes or strategies, rather than by rewriting existing methods. If old code is left untouched, it cannot develop bugs caused by edits.

#2. Stability of contracts

When a system relies on interfaces or abstractions, those contracts stay unchanged. Regressions occur when changes break interactions between components. OCP prevents such breakage: a stable contract keeps components compatible.

#3. Minimizing modifications to central logic

In scenarios that violate OCP, every new requirement forces changes to key areas of code (for example, adding new if/else branches). These are high-risk points. When OCP is followed, the central logic stays unchanged.

#4. Locality of changes

Modifications live in new modules that do not overlap with old logic. Local changes are easier to control, test, and track.

#5. Clear separation of responsibility between modules

When a module does only one thing and is extended through an interface, the chance of accidentally changing the behavior of other functions decreases. This directly reduces the risk of unintended consequences.

#6. Easier to automate testing

A new functional block can be tested separately, without touching the existing system. This reduces the chance of missing a regression, because testing becomes more transparent and predictable.

Summary: OCP reduces regressions because it reduces the number of changes to the stable part of the system and moves the extension process into new, isolated modules. The fewer edits made to old code, the lower the chance that something breaks.