Oberon is the King of the Fairies and lives outside the Athens world created by Shakespeare. You should know that Shakespeare gave this play's city scenes a classical Greek setting and drew on classical source material, but its characters are drawn from a purely Elizabethan time and place.
Although Shakespeare changed some of the common assumptions about Fairies (and the society of his day did, for the most part, believe that faires were actual real beings), he draws heavily on the beliefs of his day. I suspect that, though they saw the world as being influenced by a number of gods living on Mount Olympus, the ancient Greeks did not believe in fairies, per se.
It was quite common for Shakespeare to take an ancient or exotic setting and/or source material for one of his plays and then simply create characters and situations that were drawn completely from his Elizabethan world. Shakespeare was not interested in creating historically accurate characters or locations, but rather holding the mirror up to his audience, so that they might see themselves reflected in the behaviour of the characters they watched onstage.
So, Oberon is not an Athenian aristocrat, he's a Fairy. He may indeed be royalty (He is the King.), but he's not Athenian in any way. For that matter, one would be hard pressed to find much beyond the names of the actual aristocrats in this play -- Theseus and Hippolyta -- that bears any significant relationship to ancient Greek/Athenian behaviours. They hunt and have a wedding feast with a play for entertainment -- common past-times for upper class Elizabethan England.
Shakespeare creates the characters in Midsummer (including Oberon) as he does for all of his plays, from his experience of life as he knows it.