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1.
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!
by Jedi Master Luna August 14, 2006
 
2.
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!"
by Youwishi'dtellyou June 18, 2008