The Marvel Directory says the following about The One Above All:
"It is not known how many Celestials there are in existence. Even the number of Celestials on Earth in the Fourth Host is a mystery. Nine were known by name and function, others were glimpsed but not identified. The Fourth Host was led by the Celestial known only as The One Above All, who remained aboard the orbiting mother ship during the entire stay on Earth. The head of the landing party was Arishem."
Read it at marveldirectory.com/alienraces/celestials.htm
Also, I found a website revealing each appearance of The One Above All. Mainly, he has appeared in the Thor comics (4 times) and it has a picture of how he looks like, along with every other revealed celestial.
Go to geocities.com/brenni_au/BiosCelestials.html He is about halfway down.
Now, after reading from these sources, I think you will agree with me that it is total bullcrap that The One Above All is accually the completely omnipotent god of the Marvel universe, which even The Living Tribunal serves. In fact, there never were any signs at all that The One Above All was some omnipotent god. The name might have confused somebody, but it is clearly merely the leader of the celestials.