Sophocles expected his audience to know the myth of Oedipus and to be familiar with the members of the House of Thebes. Much of Oedipus Rex depends on irony, both situational and verbal. The audience knows the truth of Oedipus' birth and the awful circumstances in which he now lives, but he searches for knowledge through the course of the play. Despite his efforts to thwart his destiny, he has indeed killed his father and slept with his mother. These actions have already occurred when the play opens.
The Greek audience knew the plot; what they wanted to see was how Sophocles dealt with such issues as man's relationship with the gods, the results of too much pride, and lessons to be learned about how they could live their own lives.