There are just so many versions of Oberon that you could construct him as anything.
In one version he's the king of all elves. This would seem to imply he's the Erlking, as has been mentioned.
Or, since "elf" is basically just another word for "faeries," perhaps he's the king of all faeries. This would seem to make him a terribly powerful being, indeed. Or, for a nice twist, perhaps he's not actually all that powerful, but it is one of the laws of Faerie that the Queens may not disobey him in certain instances; this makes him incredibly influential, even if it doesn't necessarily grant him god-like power directly.
Or maybe he's the version that's a beautiful being of dwarf-like stature. Maybe he was once the king of elves, also King of Spring. He was married to Titania. Then, with the fall of Spring, he lost much of his power, and the Erlking snatched up the reins. This builds a nice possible selection of conflicts.