The exclamation OMG expresses astonishment, excitement, embarrassment, etc.
It is from the initial letters of oh my God (the final element may sometimes represent gosh or goodness). This initialism is older than the Internet or even the Usenet (an early computer network established in 1980), since it is first found in a letter that the British admiral John Arbuthnot Fisher (1841-1920) wrote on 9th September 1917 to the British statesman Winston Churchill (1874-1965):
Lord Fisher to the Right Hon. Winston Churchill.
My Dear Winston,
I am here for a few days longer before rejoining my “Wise men” at Victory House—
“The World forgetting,
By the World forgot!” *
but some Headlines in the newspapers have utterly upset me! Terrible!!
“The German Fleet to assist the Land operations in the Baltic.”
“Landing the German Army South of Reval.”
We are five times stronger at Sea than our enemies and here is a small Fleet that we could gobble up in a few minutes playing the great vital Sea part of landing an Army in the enemies’ rear and probably capturing the Russian Capital by Sea!
This is “Holding the ring” with a vengeance!
Are we really incapable of a big Enterprise?
I hear that a new order of Knighthood is on the tapis—O.M.G. (Oh! My God!)—Shower it on the Admiralty!!
* This is a quotation from Eloisa to Abelard (1717), by the English poet Alexander Pope (1688-1744).
But Fisher was perhaps punningly referring to the Order of Saint Michael and Saint George, which at this point had had no Grand Master or Chancellor for several years; these were appointed on 4th October 1917.
Lord Fisher was also the first known user of the expression Buggin’s turn.