Functional pattern matching (more rigorous IFs) is open for functions but closed for extension. You can write as many functions as you like, but you need to modify all of them if you add a new data type.

OO polymorphism is closed for functions but open for extension. You have a fixed number of functions (methods), but you don't need to update all the call sites if you add a new data type - merely implement all the methods.

The requirement for what needs to be open and what can be left closed is what determines which choice is better.

For example, a GUI framework is best left open for extension, because every application GUI usually ends up with specific widgets custom-designed for that app - sticking with standard widgets tends to make apps form-heavy and lacking in polish. But for the widgets to work in the framework, they need a fixed set of methods to be consistently manipulable.

A compiler AST is best left open for functions, because the majority of work on an AST is in algorithms that transform the AST. The language changes at a much slower pace than additional optimizations and analyses, and frequently new language additions can be represented in terms of existing AST constructs (where they are syntax sugar, effectively built-in macros). So having a more fixed set of AST node types is less of an issue.

Choosing one or the other on the basis of the orthodox religion of OO or functional programming, meanwhile, is just obtuse.