Interestingly, OMG (the acronym for “Oh My God”) is much older than most people would think. While associated with chatrooms and most would date it to the 90’s, it actually goes back to at least 1917 when it was used in a letter to Winston Churchill.
Sure enough the Oxford English Dictionary does list it:
1917 J. A. F. Fisher Let. 9 Sept. in Memories (1919) v. 78, I hear that a new order of Knighthood is on the tapis—O.M.G. (Oh! My God!)—Shower it on the Admiralty!
There you have it. Everything old is new again.