The structure of Greek drama follows a pattern, as do the plots themselves. The Greek audience would know from the beginning that the tragic character was fighting a fate that could not be overcome, for these stories were not original except in their specific interpretation or characterization. The nature of the stories was never altered. The audience at The Oresteia knew what would happen and so was already prepared for the lack of freedom of action facing these characters. The audience saw a lesson in this, a caution against excessive pride in lives actually controlled by the gods and their beneficence or lack of it. The story of Oedipus as told by Sophocles was well-known to the audience, and the playwright did not have to explain the ramifications of the story to them. What they would see unfold before them was the way in which Oedipus learned of his crime and the aftermath of that knowledge.
Oedipus is well-established as an important character--he is the ruler of Thebes and is responsible for the welfare of the people. What happens to him thus has import beyond his immediate and personal situation--tragic characters are also high-born, as Aristotle says, because what happens to them has greater meaning than what happens to the low-born. The fatal flaw in Oedipus is that he did not heed the gods when they spoke through the Oracle at Delphi, telling his parents (and indirectly, him) that he would one day kill his father and marry his mother. Oedipus knows of this prophecy, and yet he does kill a man and marry the man's older widow. Oedipus thinks that Polybos and his wife are his parents, which they are not, and he clearly has no intention of committing the crime that he does commit. He discovers his crime as he questions various people about the troubles visited upon Thebes and about his own past, learning the identity of Iocaste and of the man he killed. He cannot face the reality of this knowledge and suffers ...