Yes, as Jil said, it's essentially the same as "hey".
"Oi, mate!" is a call designed to get someone's attention, and would normally be followed by some sort of instruction, eg, "oi mate, come over here a minute!", or "oi mate, get out of my way!".
It could be Australian, but could just as easily be Cockney English, especially if you end the "mate" with a nice glottal stop, instead of the "t" :-)
There's even a Wikipedia article dedicated to the subject (!): http://en.wikipedia.org/wiki/Oi