The Open Close Principle states that the design and writing of the code should be done in a way that new functionality should be added with minimum changes in the existing code. The design should be done in a way to allow the adding of new functionality as new classes, keeping as much as possible existing code unchanged. Software entities like classes, modules and functions should be open for extension but closed for modifications.
Like every principle Open Close Principle is only a principle. Making a flexible design involves additional time and effort spent for it and it introduce new level of abstraction increasing the complexity of the code. So this principle should be applied in those area which are most likely to be changed. There are many design patterns that help us to extend code without changing it, for example decorator.