He also said that throwing unbelievers into fiery hell for an eternity was justified because "God is above all things just", which prompted me to end the discussion
I love it when someone accidentally tells the truth. 'god is above all things just' seems like another way of saying that 'Justice is irrelevant to god,' which, if one looks at Biblegod's career as a whole, is certainly true.
But to your question:
If god is truly omnipotent, it seems to me that omnipresent ought to be a feature of omnipotence. At least within our space/time continuum. Otherwise it's like this: 'god is ominipotent, but he isn't here today...so, let's PAAAARRRRRRRRR-TAY!'[cue Beastie Boys record]
Of course, I think the whole notion of an anthropomorphic invisible sky wizard being the 3 O's is pretty unlikely, so the person you were debating might have been bumping up against that truth and so had to back away.
Incidentally, aren't there 4 O's:
Omnipotent, Omnipresent, Omniscient, and Omnibenevolent?
But since god is above things just, he gets exempted from the duty of omnibenevolence and so he gets to PAAAARRRRRRRRR-TAY!'[cue Beastie Boys record]