However, when you think about it, it makes perfect sense that someone in the navy would create something like this: In the days before VHF and single-sideband radio, ships' primary form of communication was Morse code -- a system that, like early texting, required each letter to be painfully spelled out. Navy men were used to abbreviations, and thus likely to come up with some of their own.
They also invented sexting.
And, since "Oh my God" is an entirely pointless statement, it also makes sense that it wouldn't really catch on among Fisher's peers. Valley girls, on the other hand, are all about pointless statements and uneconomical language (thanks, as we've already pointed out, to Frank Zappa), so this was a natural addition to their lexicon.
As for how they found out about it -- we'd like to think that there had to be at least one teenager in the eighties who was really into obscure documents from early 20th century British naval history.
Sadly, a bullying accident in 10th grade left him permanently swirlied.