When someone, usually
British or
Australian, yells in either anger, excitement, or
fear. It generally needs context, and is usually said in the beginning of a sentence.
Anger: "Oi
mate, you done steal my boat!"
Excitement: "Oi, look what you've done, mate! Now we can go back to Ireland!"
Fear: "Oi, I think that
big croc is gonna get us."