[@dragonmancer]My idea for the "eldest" thing is that he remembers a time before any of the other (currently present) gods existed. Something like a "first generation" god, and is perhaps the last member of their kin.

The omnipotence would work exactly like this: he knows everything. He doesn't know the future, but every piece of information that currently exists in the world is in his possession.

Though he's generally an observer, he takes action when he deems it necessary, and acts swiftly and decisively. Perhaps at some point in the past he acted by "taking the mind" of another god, essentially lobotomizing them.