letter written by Lord Fisher to Winston Churchill on 9th September 1917
image: AbeBooks book blog
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):
I hear that a new order of Knighthood is on the tapis—O.M.G. (Oh! My God!)—Shower it on the Admiralty!!
But he 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.