The sound British people make when trying to get your attention. Typically when they're angry they will speed walk towards you and yell "Oi!" at the top of their lungs.
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."