In Oedipus Rex, what is the reversal of this play?
It all depends on your specific definition of the term. For if you simply mean once the character's fortunes change for the worse, then one might consider the very beginning of the play when Oedipus sends Creon to the Oracle at Delphi for an explanation of the plague and the horrible conditions of the city of Thebes. Once Creon returns with the answers and the knowledge that Oedipus is responsible, Creon requests that they speak in private:
Speak out to all. I sorrow more for them than for the woe which touches me alone. (Prologue 99-100)
Considering that Creon and Oedipus could have known his misfortune privately, it starts a public fallout for which there is no return. The second reversal of fortune is when Oedipus summons Tiresias and since Tiresias knows the situation, he had tried to make himself forget. Oedipus and Tiresias end up arguing with Oedipus accusing Tiresias of being in a conspiracy with Creon. The reversals are almost endless.