The Greek drama Oedipus Rex is clearly a tragedy. It definitely meets the five main criteria for a tragedy: a tragic hero of noble birth, a tragic flaw, a fall from grace, a moment of remorse, and catharsis.
Oedipus Rex clearly meets the first of these five criteria. Oedipus is the son of Laius, who was king of Thebes. Even at the beginning of the story, when we are told that Oedipus is the son of Polybus, he is still of noble birth; Polybus is king of Corinth.
The tragic flaw, or mistake that a character makes, in Oedipus Rex does not actually take place during the story. We only watch as Oedipus and the rest of the characters discover this mistake that was actually made long, long ago and cannot be reversed. This tragic flaw is of course Oedipus killing his father Lauis, and then marrying Jocasta, his mother. We realize that these actions have taken place much earlier in the story than the characters do. However, both of these events actually took many years ago.
The fall from grace in Oedipus Rex is when Oedipus, Jocasta, and all the other characters in the story realize that Oedipus actually did murder Laius and that Jocasta is indeed his mother as well as his wife. This occurs rather quickly, very close to the end of the play.
The audience sees this coming long before it actually does, however. In one of the passages of Oedipus speaking with Jocasta, just about everything is spelled out for us. Jocasta speaks of Laius leaving the castle with just a few servants and his being killed where three roads meet. Oedipus claims that he killed somebody where three roads met, who had a few servants with him. As though this isn't enough, Jocasta describes Laius to Oedipus by saying "his figure was not much unlike your own" (p. 27). Oedipus, after hearing all this, says "O, it is plain already!" (p. 27) indicating that he was the killer of his father. He goes on to make absolutely sure, even though it is obvious that he was...