An expression like Oh My God, but dirived from the phrase OMG. SO, pronouced like the phrase, like oh em jee. But then it kinda turned into omigee, so that's how you say it.
It pretty much means the same as Oh my God, just. . .yeah. . .
"Omigee, he did NOT just say that!"
1) an exclamation of extreme excitment, amazement or disgust. much like oh my god except 'god' is replaced with 'gee' pronounced the same as in 'geese'.
2) Can be used in a very sarcastic manner, or to mock someone or something
1) omigee i am so fucking happy i got my tickets to the IKTPQ show!!!
2) wow. omigee. my chemical romance is like, sooooo unbelievably cool *rolls eyes*