"He is above me."
Translation:Er ist über mir.
oben = "at the top" -- it's an adverb, not a preposition, so you can't follow it with a noun or pronoun just as you can't say "he is at the top me".
über is a preposition ("above, over") and so that is what you have to use here to make über mir "above me, over me".
It can mean that in a context such as "walk across the street" (über die Straße laufen), with the accusative as a destination of motion.
But with the dative of location, it means "above, over".
does it mean, that he is my boss, or that his apartment is above mine?