Like "LOL," "WTF" and other similar abbreviations, "OMG" gained widespread use in the last years of the 20th century with the advent of instant messaging -- but it actually goes back much, much farther than that.
But It Came From ...
According to the Oxford English Dictionary, the first person to use "OMG" was a 75-year-old British admiral, which is about as far as a carbon-based life form can get from a teenage valley girl in a mall texting her friends about Justin Bieber's butt. His name was John Arbuthnot "Jacky" Fisher, 1st Baron Fisher of Kilverstone, and he coined the term while writing his memoirs ... in 1917. Almost 100 years ago.
Via Wikimedia Commons
Another chapter is just John Mayer lyrics written out in semaphore.
The exact phrase was: "I hear that a new order of Knighthood is on the tapia -- O.M.G (Oh! My God!) -- Shower it on the Admiralty!" (his subtle way of hinting that he hoped to be knighted). It's ironic, then, that one of the most popular abbreviations in the world was created by someone who didn't quite grasp the concept of using the acronyms to save time, since he immediately followed the term with the phrase it was supposed to shorten.