I always thought it was sort of an expression of exasperation, but here, in this videotaped (inadvertently) fistfght between a runway model/actor Antonio Sabato Jr. and a designer’s assistant, it’s used in the place of an escalating expression of alarm like “Hey! Hey!! Hey!!!”
Fists of Fashion
The Yiddish Oy is a cry of dispair. The British Oy is to get someone’s attention.
Haj
The impression I get from British and Irish-influenced punk music, as well as the soccer episode of the Simpsons (in which Groundskeeper Willie and a bunch of Scottish thugs jump into the soccer riot murmuring “Oi! Oi!”) is that it’s sort of an all-purpose interjection, used to express excitement, anger or other high-running emotions.
“Oi” can mean just about anything. “Hey you!”, “Ouch”, “That’s not fair”, …
Let me paint you a picture. It’s boat race night. A chap has got a little squiffy after downing too many cocktails at his club (whether in celebration or to ease the pain of defeat really makes no difference). If said chap, on the perilous journey home, should take it upon himself as his civic duty to knock a helmet from a policeman’s head, the policeman would be entirely within his rights to cry “oi!” - inasmuch as to protest, “I say, what are you up to?”
If the assault continues relentlessly, perhaps with the chap then cuddling up to the policeman and beginning to sing about his mother, a repeated and increasingly loud “oi” would be entirely appropriate.
Essentially its use here is to express one’s consternation at the behaviour of another chap. However it has plenty of other uses, and one can only really gain a proper sense of its meaning by spending a lot of time around those who use it often. Boat race night this year has passed us by, I’m afraid, but one needn’t be tied to one date. Be free with your tempo, what?
It also means “hey!” in Japanese, but it’s considered pretty rude.
…and the dyslexic jewish rapper who greets you with an ‘Oy”. 
In Australian, “Hey!” as well.
AC/DC’s “TNT… Oy! Oy! Oy!” comes to mind.