Although the word 'oder' does in fact mean 'or', there is a sneaky second meaning that often creeps up in spoken german. When thrown on the end of a sentence with a question mark, it turns the sentence in to a sort of question, confirming the statement just been said.
Heute ist Dienstag, oder? (It's Tuesday today, right?=)
Wir haben heute eine Prüfung, oder? (We have an exam today, right?)
In other words, 'oder?', with and only with the question mark, means 'right?'.
So beware when you see the word 'oder' with a question mark. Although, it's not hard once you get used to it...oder? :)
That's really not any different than an English speaker adding "right" at the end of a sentence, turning it into a question. There's a reputation that Canadians say "eh" at the end of a sentence in the same manner (although I almost never hear it myself).