Functional or OOP what's the big difference?

There's way more to the differences between OOP and FP than how you call functions/methods, and when you boil it all down it's really all to do with how your code and data is organized.

If we ignore the fact that most languages misunderstand the original idea of object orientation, OOP tries very hard to bundle code and data together in separated units so that all the code that has to do with some data is passed around with that data.

When you create an object of that type you get all the methods you need to work with that object bundled alongside it, but you also get all the methods you aren't going to use, and if you need to do something that the object doesn't already know how to do then you get into the territory of subclassing or extending/monkeypatching that object.

In Functional Programming, there is a bigger separation of code and data (unless you're using a LISP, but let's not complicate matters). You have some data that you can pass around in your program, and then you have some code (functions) that operate on some data. The code is not tied to some particular data, it is independent. If we look at a language like Python for example you have the function len(). It's not coupled to any object (despite Python being heavily OOP), instead you can call len() on literally anything and for nearly every piece of data you throw at it len() will respond with a length/size that is appropriate for the type of data you provided it with.

This comment on StackOverflow illustrates when it would be a good thing to choose one over another, and why multi-paradigm languages (Python, Nim, JavaScript) are generally more ergonomic than single-paradigm languages are since they let you freely choose whichever style fits the current problem instead of trying to shoehorn your solution into whatever paradigm. That's where all the problems come from where you invent convoluted patterns for OOP to solve issues that are trivially solved if you weren't forced to make everything a class (C#, Java), or where you get into hairy mathematical concepts like Monads to circumvent your language not wanting you do make functions with side-effects such as printing some text to the console (Haskell).

Programming languages and software in general exist to solve problems, they should get out of your way and help you to make things easier. Not force you to tricks and hacks.

PS.
If you look at a language like Nim you'll see why it's not useful to look at how things are called to determine the differences in paradigms:

type Vector = tuple[x, y: int]

proc add(a, b: Vector): Vector =
  (a.x + b.x, a.y + b.y)

let
  v1 = (x: -1, y: 4)
  v2 = (x: 5, y: -2)

echo add(v1, v2) # "functional" style
echo v1.add(v2) # OOP style
echo v1.add v2 # Don't even need parenthesis if it's unambiguous

Yup I see
OOP is widely used in production since JAVA dominated the industry in the late 90's

Well crafted answer
And it explains really well
Thank you