Used when you or someone else does something stupid. It also can be used if one is annoyed, disgusted, bored, surprised, or in pain. Or just for the thrill of saying something nobody else says.

(sense 1): Interjection signifying pain, weariness, disgust, resignation, etc. Very often part of oy vey or oy vas mir. Compare to English Jeez. (from Yiddish "oy", probably from old German; probably cognate to English "oh").

(sense 2): Interjection signifying annoyance. Very similar to sense 1, but more annoyed than resigned. (Probably from Cockney accent of English "Hey".)

(sense 3) Greeting meaning "hello" or "Hi". (Probably from Cockney "hi").

1.) an expression of shock or amusement; oh my

an exclamation of suprise, dismay, confusion, or any other feeling (from Heberew)

When you're so horny that the only thing that you can say is "Oy!"

"the funniest word ever" according to Lorelai on Gilmore Girls.

A term used between teens of DDP (Drunk Disorderly Punks) as a way of hollering at each other and saying "hi", since they are usually wandering the streets and need to yell to get attention. It also serves a double purpose of letting them know the one who yelled it is one of them.