an exultation expressing extreme excitement or surprise in a particularly pleasing situation; can also be used to replace the phrase "holy shit!"
Thing 1: Have you seen "The Shawshank Redemption"?
Thing 2: "OMGAH! I LOOOOVE that movie!"
Used in MSN conversations, when you see/read/whatever something that is really hot/cute/sweet/nice etc.
MSNer 1: Hey! Check out this superhawt RyRo picture (:
MSNer 2: OMGAH! He's so cute!