Used to express extreme surprise. Meaning, Oh my fucking god BBQ. The BBQ is added at the end to lengthen the acronym and make the statement more meaningful.
-OMFGYB!!, how could you fuck my boyfriend?
-what? it wasnt all that great anyways.
-really ??
-yea you can fuck him now
-oh ok sry bout the omfgyb
-np
omfgtfo is the combination of "omfg" and "gtfo" in which the "god" and "get" are shared resulting in "oh my fucking godget the fuck out". This phrase emphasizes the surprise of the person saying it.