OMFJ simply means Oh my Fucking Jelly, Stronger version of OMJ
Guy 1: look i drew all of this in 5 minutes only.
Guy 2: really? that's amazing, OMFJ! i envy you
OMFG is a more dramatic version of OMG, (Oh my fuckin god) it can also refer to another meaning: Oh my forgiving god
Guy 1: hey dude have you seen my new car? it's fucking amazing
Guy 2: yes, OMFG that's one nice car indeed