Chat-Speak for "Oh! Hi There!".
1.Used to greet someone that you haven't seen in a while, or haven't ever met before.
2.Also used when you suddenly notice someone/living thing (greeting).
Bob: OHAIDAR, WHERE HAVE YOU BEEN FRANK?
Frank: WITH YOUR MOTHER. :)
Bob: HEY LOOK! ITS A KITTY-CAT!
Frank: OHAIDAR KITTY-CAT.