Oi or oy has many meanings/origins. In Britain, it's like "Hey!", usually in a negative situation when you are angrily trying to get someone's attention, or when you're indicating you're offended by something someone just said. In Yiddish, it's like a lament. "Oi, this sucks." In music, usually ska, it's a kind of chant that people do. "Oi, Oi, Oi, Oi!" I have also heard that to some people this can be considered some sort of racist remark, though I don't know to what people this supposedly refers or who supposedly uses it.