In my experience, 'oi' has a few usages:

- Drawn out "oiiiiiii" and a pointed finger when someone makes a witty remark or has some kind of success. As in, "oiiii, nice one".

- "OI!" as a generic "hey!" shouted at someone to get their attention in the street.

- "oi?" with an upwards inflection to express disbelief

- Getting your friend's attention, usually followed by an insulting term like 'dickhead'. As in, "oi dickhead, watch out" when someone is trying to pass them on the street.