Oy

[Little_Girl]

(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).

[Dave Paprocki]

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

[Jason Yan]

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

[Luis Fok]

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.