One of the major themes of Oedipus Rex is the blindness of Oedipus. The irony, of course, is that the blind seer, Tiresias is able to see whereas Oedipus, who can physically see, is actually blind. As the work progresses, it becomes increasingly clear to all the characters that the plague of Thebes is due to the transgression of Oedipus - he is the one who unwittingly killed his father and married his mother.
What makes Oedipus's blindness most pronounced is the fact that he is such an able person. For example, he was able to solve the riddle of the sphinx. Oedipus knows his self worth and his talents. This pride prohibits him from seeing the truth. When the chorus at the end of play sees this, they speaks these words:
"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."
In light of these words, we can say that it was preeminently Oedipus's pride that led to his downfall on account of his blindness.