"He is above me."
Translation:Er ist über mir.
Why is "mir" being used here instead of "mich"?
Because über is one of those prepositions that can take either the dative or the accusative case, and uses the dative case to indicate location but the accusative case to indicate destination-of-motion.
And here, "above me" describes a location, so you need the dative case: über mir.
über mich would indicate that "(to a point) above me" is the endpoint of some motion, but "to be" is not a verb of motion. (The motion may actually continue past the point above you -- der Ball flog über mich "the ball flew over me / over my head" indicates that the ball started somewhere, flew until it was over your head, and then continued flying -- since it can't stop in mid-air.)
this link has a VERY helpfull sheet for me (about cases) and i hope also for you https://www.reddit.com/r/German/comments/277y3g/german_cases_cheatsheet_resubmit/
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".