In the simplest terms, Oedipus is a man who thinks he knows who he is, but clearly he does not. Part of that, of course, was perpetrated on him by deception; even worse, it was his parents who deceived him.
Oedipus thought he was the natural son of a king, the king of Corinth. In fact, of course, he is the son of Laius, king of Thebes. He thought he was destined to kill his father and marry his mother, so he left and believed he would avert his fate as a murderer and an incestuous man.
He believes himself to be a prince and above those sins, yet he continues his journey and commits patricide and incest. What he thought he was not, he became.
As king, Oedipus saw himself as a father and husband and brother-in-law. Instead, he is a father/brother, husband/son, and brother-in-law/nephew. He sees himself as protector of his people, when in fact he is the cause of their curse.
The tragedy in Oedipus is the fall of this man; he falls because he is not who or what he thinks he is.