The tragic downfall of Oedipus in Sophocles's Oedipus Rex is Oedipus's belief, based on his overwhelming pride (what the ancient Greeks called hubris),that by exercising his own free will, he can thwart the will of the gods and choose his own destiny.

Everything that the gods preordained would happen to Oedipus happened long before the play begins.The gods determined that Oedipus would kill his own father. He did. The gods further determined that Oedipus would marry his own mother. He did.

Oedipus unknowingly killed his own father, Laius, on his way to Thebes from Delphi, where Oedipus learned from the oracle that he was destined to kill his father and marry his mother. As fate would have it, Laius was on his way to Delphi to inquire about omens foreshadowing his death.

After he killed Laius, Oedipus continued to Thebes, where he solved the riddle of the Sphinx, for which he was awarded the throne of Thebes and marriage to Queen Jocasta, who Oedipus didn't know was his own mother.

There's nothing that Oedipus can do to change these events. Oedipus and everyone involved with Oedipus's life brought these events about by attempting to circumvent the will of the gods, either unknowingly or by pridefully exercising what they believe is their free will.

The only things left for Oedipus to do in Oedipus Rex are to discover his tragic flaw that influenced his own choices, to learn about choices made on his behalf that led him to this point in his life, and to suffer the fate that the gods have decided to impose on him for the defiance of their will.

To force the issue, the gods bring a plague and famine on Thebes and give word through the oracle that the plague and famine are a result of Laius's murderer still living among the people of Thebes. Oedipus, the self-proclaimed "world-renowned king," takes upon himself the responsibility of finding Laius's murderer and of applying the appropriate punishment—"Banishment, or the shedding blood for blood"—to appease the gods and relieve the people of Thebes from their suffering.

Oedipus has ample opportunity to display his tragic flaw by defiantly rejecting any suggestion that he himself is Laius's murderer or that he's at fault for any of his own choices to defy the gods.

Ultimately, Oedipus is revealed as Laius's murderer, and he's exposed as a person whose prideful decision to defy the gods on the road from Delphi set in motion the events that led to the fulfillment of the oracle's prophecies and to the suffering of the people of Thebes.

Oedipus administers his own punishment. He blinds himself—"shedding blood for blood"—and banishes himself from Thebes, thereby acceding, finally, to the will of the gods and fulfilling his own destiny.

The lesson of Oedipus's life, as Sophocles presents it to his audience in Oedipus Rex, is voiced by Creon:

CREON. Crave not dominance in all,
For the mastery [pride] that raised thee was thy bane and wrought thy fall.

Like any story from Greek drama, the downfall of Oedipus is tied to his tragic flaw. Greek playwrights strove to create protagonists who were mostly good people but were still afflicted with a tragic flaw, also known as a harmatia. For Oedipus, this flaw is his inflated sense of pride, or hubris. Oedipus certainly thinks highly of himself. He has solved the riddle of the Sphinx and saved the city of Thebes, becoming its king. These accomplishments are enough to go to anyone's head, and Oedipus is no exception.

Even before he became the hero of Thebes, it seems that Oedipus already suffered from hubris. This is what caused him to attempt to defy the fate of the gods. Earlier in life, he felt that he could circumvent the prophecy that he will kill his father and marry his mother simply by running away from his home in Corinth. A recurring theme in many ancient Greek stories is the downfall of a heroic man who attempts to defy the will of fate.

Furthermore, Oedipus's pride makes him unable to see what is clear to everyone else. When Creon presents himself to Oedipus as the voice of reason, Oedipus grows suspicious. He does not like Creon's message and accuses him of trying to usurp the throne. When the blind soothsayer Teiresias correctly identifies Oedipus as the cause of the plague, the king accuses him of treason. As more and more evidence is presented to Oedipus that he is the murderer of King Laius, Oedipus refuses to accept it until it is far too late.

In Sophocles's famous Greek tragedy, King Oedipus's tragic flaw is his hubris, or excessive pride, which leads to his downfall. In the play, Oedipus is destined to kill his father and marry his mother but does not recognize his origins or background. When a terrible plague strikes Thebes, the oracle of Apollo at Delphi states that the murderer of Laius remains in the city and that the plague will endure until the murderer is expelled. As King of Thebes, Oedipus falls victim to his hubris by ignorantly dismissing the prophet Teiresias's message, denying the truth, and accusing Creon of colluding with the blind seer to usurp power. Oedipus's hubris prevents him from recognizing the truth that the prophecy was fulfilled.

As a young man, Oedipus grew up in Corinth and believed that King Polybus and Queen Merope were his biological parents. After consulting the oracle as a young man, he discovered that he was fated to kill his father and marry his mother. In an attempt to change his fortune and deny fate, Oedipus fled Corinth and murdered Laius on his way to Thebes. Oedipus's hubris is demonstrated by his attempt to defy the will of the gods by altering his destiny. Oedipus was completely unaware that Laius was his biological father and proceeded to marry his mother, Jocasta, after arriving in Thebes and solving the riddle of the Sphinx. Toward the end of the play, Oedipus recognizes that he could not avoid his fate and that the prophecy was fulfilled, which influences him to blind himself and go into exile.

As said in the other answer, Oedipus's tragic flaw is his hubris, or pride. Although he is fated to kill his father, his pride is the catalyst for that death; in his rage that Laius's caravan won't give way for him on the road, he kills Laius, his father.

But the bigger picture is that Oedipus's hubris blinds him. He simply doesn't think he can do anything wrong. Therefore, when he is calling down his wrath on the person whose sin has caused the plague in Thebes, it never once occurs to him that he could be the problem. It is only later that it dawns on him that he is the source of the plague because he has unknowingly murdered his father and married his mother. Only after he physically blinds himself does he, paradoxically, gain insight. The blinding is an act of anguish but also of humility; it is a physical representation of his understanding of his own limits, an acknowledgment of the ways he has been blind inside.

Most scholars agree that Oedipus's tragic flaw is what the ancient Greeks called hubris, or overweening pride. This pride manifests itself when Oedipus encounters his birth father Laius at an abandoned crossroads. Oedipus, being full of kingly pride, refuses to give way. In the ensuing scuffle, he kills his father—although at this point in the play he doesn't know that Laius really is his father. Nevertheless, Oedipus's murder of Laius sets in train a series of events that culminates in his tragic downfall.

Though it's important to note that the great philosopher Aristotle—who considered Oedipus Rex to be the perfect tragedy—held that Oedipus's downfall was caused by a mistake rather than a fatal flaw as such. Whichever way we look at the matter, there can be no doubt that Oedipus's actions on that fateful day at the crossroads lead directly to his ultimate downfall.