According to Aristotle, tragic heroes suffer from a fatal flaw, otherwise known as hamartia, that is the root of their downfall. Often, this fatal flaw is arrogance, or a belief in their own abilities that turns out to be misguided or misplaced. Arguably, this arrogance and overconfidence is the tragic flaw in the character of Oedipus, as he believes that he can be the liberator of Thebes once again, and he sees himself as the solution to their problem. Note what he says about himself in the following quote:
I'll start again--I'll bring it all to light myself! ...
Now you have me to fight for you, you'll see:
I am the land's avenger by all rights,
and Apollo's champion too.
Oedipus therefore displays considerable overconfidence in his own abilities, ironically not realising that he is both "the land's avenger," but also the cause of this present calamity. He will indeed bring it all to light" himself, and through doing so will realise his own hubris and blindness as to his own identity. The tragic flaw of Oedipus is therefore his sense of belief in himself and in his own abilities. He describes himself as the saviour of Thebes whereas in reality he is the cause of the present problems.