O = Open/Closed Principle
This is all about being “open for extension but closed for modification”. Your code should be extendable. To the point where you don’t need to constantly change anything about it. This can come in many forms; for example instead of overloading a class with a number of different methods. Such as the “Person” class needing a method like “write a book” and also like “fight a fire”, or even “cook a five star meal”. Instead we could separate these into classes that all inherit from the “person” class. Which would allow use to write code to extend existing functionality. Without having us go in and make changes to the core logic of the person class.
Another example of this is when we are using a large “if – else” or “switch” statement. And we do things based on what input we get passed in. Instead of having this large “if -statement” we can refactor the logic back into the input we are are being passed in. If we had gotten a generic account class as an input. And we had to calculate their net-worth based on their account type. We should create classes for each account type, and store the calculation logic in them, rather then in the “if else” clauses.