it was said in Phoenix Endsong, when the phoenix came to find jean grey, she felt a omega level mutant and went to quentin quire, also beast, and emma frost stated he was an omega level mutant, when the shiar tried to kill jean grey they also tried to kill quentin quire saying all omega level beings that can host the phoenix need to die 

I could have sworn i had updated this page with the correct issues, are there several similar pages on the subject?
Anyway Rachel was the first confirmed omega, forget what issue number, but it was in Uncanny way back when X-Men fought Noimrod.
Maybe Uncanny 207??

Noimrod, huh :-)

That was off-panel right before they fought Nimrod :p

"That was off-panel right before they fought Nimrod :p
"

I must have updated some dupe page with that info since i don't have any points on this page. Anyway i checked up on it and it was Uncanny 208