So in theory you only use and once for the final item in a list. This isn't strictly true, but using and indicate that it is the final item.

So a sentence without the Oxford comma: "I like apples, bananas and oranges."

A sentence with the Oxford comma: "I like apples, bananas, and oranges."

You might be thinking, "they read the same way so why does it matter?" Well, think of the ambiguity of the first sentences. Do you like bananas and oranges separately? Or do you only like them when they're together? Without the Oxford comma we don't know.

Case in point: "I invited Jack, Peter and Mary to my party." vs "I invited Jack, Peter, and Mary to my party."

Are Peter and Mary a couple? Were they invited together? Only the first one raises these questions.

Additionally, use of the Oxford comma lets you do this, "I invited Jack, Peter and Mary, and Daniel to my party."

Remember how I said you typically used and once to mark the final item in the list? Well now you can use it multiple times to indicate things that go together in the list rather than separate items. In this case, it implies that Peter and Mary were indeed invited together. You have to be a little careful with as it can sound weird, but it works.

Note that this is only for lists of three or more items. It isn't needed for only two things.

TL;DR Not using the Oxford comma is less versatile and more ambiguous, but an Oxford comma is never wrong. Just always put a comma before the final "and" in a list.