Etymology An abbreviation of oh my God, although sometimes glossed using minced oaths to avoid offense. In its first attestation, also intended as a punning reference to the Order of St Michael and St George.
1
Internet An exclamation of excitement, surprise, shock etc.; oh my God
OMG, I just won tickets to a concert! Today is the best day ever!
…and then he said I was ugly, so I punched him in the face. — OMG, really?
2 more examples
I hear that a new order of Knighthood is on the tapis—O.M.G. (Oh! My God!)—Shower it on the Admiralty!!1917 September 9, John A.F. Fisher, letter to Winston Churchill:
10 Bands, 50 bands, 100 bands, fuck it, man / Let's just not even discuss it, man / OMG, niggas sleep, I ain't trippin', I'ma let 'em sleep2015, “10 Bands”, in If You’re Reading This It’s Too Late, performed by Drake:
Internet, Alternative, Slang Alternative form of OMG
Then, in a staggering display of empathy for the deceased lacking, this friend segues to the narcissist nub of the matter: "Omg wat if yu get arrested b4 yur bdai."2009 March 10, Rosie DiManno, “Murder reduced to infantile online postings”, in Toronto Star, archived from the original on 13 Mar 2009: