Tiresias replies that he is beholden to noone but apollo. This allows him to speak the truth to Oedipus. What caused Tiresias to change his mind and speak the truth?

At first Tiresias doesn’t want to tell Oedipus the truth. It was a, "You can't handle the truth!" sort of thing, except without Jack Nicholson. Oedipus is an arrogant jerk so he chastises the old blind prophet. Come on, who yells at an old blind man? Tiresias changes his mind and lets Oedipus have it. Tiresias tells Oedipus that he is his father's real killer and that he "made it" with his own mother. Oedipus, of course, doesn't believe this and accuses the prophet of being in line with Creon.