how does Oedipus achieve greatness at the end of the play, despite his sins? (Oedipus the King)

Oedipus does find redemption in the end. Perhaps the Chorus puts it best:

People of Thebes, my countrymen, look on Oedipus. He solved the famous riddle with his brilliance, he rose to power, a man beyond all power. Who could behold his greatness without envy? Now what a black sea of terror has overwhelmed him. Now as we keep our watch and wait the final day, count no man happy till he dies, free of pain at last.