(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").