(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").
OY is a better, calmer, less boomer version of PS.
It means "Oh yeah..."
As in, I want to tell you this other thing that belongs in the email, but it is not really related to the email, so I am going to jam it in at that bottom.
OY is a gang in Harlem particularly in Sugar Hill that is most known for their drill music. They have popular rappers like SugarHill Keem, Sugarhill Ddot, DD Osama, Edot Baby, Dudeylo, OY Quan, and more. OY has beef with the YGZ, DOA, and more. Their younger rappers started getting a lot more recognition after Notti Osama's(14yr old rapper who was one of the best in the scene) death. They started around crip descent unlike the YGz who started with blood descent.