To express suprise. It is the combination of "Oh", "My" and "God" to form an incoherant word. Usually used online since saying it outloud does not hold the same meaning, because people cant tell you meshed the words together.
Spackerdave: Dude, i just f*cked your mom.
ReZ: ohmygawd
An absolutely foolproof way to keep minors from viewing content deemed "too mature" for them to watch.
Billy: "So I was trying to watch some videos for an M rated game, and I ran into an age gate, so I put in my real birthday. SOMEHOW it managed to figure out that I am 12. CURSE YOU AGE GATE!!!"
Bill: "Fucking age gate! *quick scroll* THERE! I was born on January 1, 1904. Fuck you age gate."