Using Functional Programming Style in Object Oriented Languages

Using functional programming style is possible with an object oriented language while the language doesn't help you to remain declarative only in side effect free. You could use Java or C sharp or another imperative language or even a hybrid language like Kotlin for example and and accomplish that. How do you watch out for things like reassignment of variables and let's say collections collections in the standard libraries are definitely effectful? I think many mainstream languages since many years get functional programming language features so it becomes easier to write a functional code but nothing can be solved without discipline.