Oberon is the King of all Fae and the father of all Firstborn from which all the races of Faeries. His first wife is Maeve, and his second wife is Titania. Together they are the Three Creators of Faerie. He and his wives are currently missing, having sealed the deeper realms of faerie before their disappearance, 500 years ago.
Oberon's last direct reference in the series is in The Fixed Stars, in the year 572.
The Luidaeg revealed that the actions of Amandine's mother is the reason why he and his wives had disappeared from Faerie, and that the task of the Dochas Sidhe is to bring them home. This was partly why Oberon bound Evening from hurting the descendants he claimed, well aware that his own eldest daughter would relish in her psychopathic tendencies once they left.
Quotes
"Oberon doesn’t claim most of his descendants, leaving them to the mercies of their mothers. Those few races that he does claim . . . those are Oberon’s children. And Oberon’s children are heroes”.[1]