Said as 'OH MY FUCKING GOD' but the Z is silent ... This is due to the fact that OMFG was supposed to be typed, but the typer has held down the SHIFT button as well as hitting the Z button which is next to it by accident.
It is now so popular that ppl will type ZOMG on purpose for the 1337 factor :)
Jase: ZOMFG! did u see that!?
Daniel: yer man .. pwned it