I believe Tireasias was too afraid to tell Oedipus the truth about his identity. Oedipus is known to have a violent temper, and that he has a short fuse. Tiresias was scared that if he were the one to bear the news, that Oedipus might kill him, because he already killed a man because he was angry and too proud. The truth is that Oedipus unknowingly killed his father and then married his own mother.