Past my edit window, but I want to add this - I feel that I introduced some confusion by missing one magic word in one special place. The first sentence of the last paragraph should be:
That said, if you go for functional style in OOP, i.e. shoving as much as you can into stateless static helper functions and most of the rest into dumb private stateless functions, (...)
Of course I do not mean you should abandon objects where there is a strong connection between a set of data items and operations that work on them, or where polymorphism is a right abstraction. But from my experience, quite a lot of code is made through transformations applied on simple data, and when you write that kind of code in a functional style (whether as static methods grouped in helper classes, or private methods within an implementation of your class), both quality and testability rises in lockstep. And my point is that quite a lot of code can be written this way even in an OOP project.
That said, if you go for functional style in OOP, i.e. shoving as much as you can into stateless static helper functions and most of the rest into dumb private stateless functions, (...)
Of course I do not mean you should abandon objects where there is a strong connection between a set of data items and operations that work on them, or where polymorphism is a right abstraction. But from my experience, quite a lot of code is made through transformations applied on simple data, and when you write that kind of code in a functional style (whether as static methods grouped in helper classes, or private methods within an implementation of your class), both quality and testability rises in lockstep. And my point is that quite a lot of code can be written this way even in an OOP project.