Why would the story of Oedipus seem probable to the audience in fifth century, B.C.?
Known as the Golden Age of Athens, the time of Sophocles saw the naissance of democracy and the flowering of cultural entertainments such as festivals and the theater. Rhetoric was also at its height with men such as Pericles; therefore, Oedipus Rex is truly a timely drama, and its setting of a plague is also relevant as Athens endured one from 430-29 A.D. Further, the Athenians were proud of their defeat over Xerxes of the Persians, who threatened their democracy.
Oedipus Rex would certainly have seemed credible to the Athenians, who were very receptive to the format of the play since this combination of song and drama was traditional for them in theatrical performances as well as festivals. In addition, the Athenians would have been receptive to the belief in inexorable fate and the influence of the gods upon man's destiny. Moreover, the plot of Oedipus Rex has been called one of the most perfect dramatic plots ever devised, so Athenians and all that follow it are inspired. For, speeches by Oedipus at times have a larger meaning for the audience than for Oedipus. When, for example, he argues with Teiresias, Oedipus says of Kreon,
He has bought this decrepit fortune-teller, this
Collector of dirty pennies, this prophet fraud--
Why, he is no more clairvoyant than I am!
Of course, the audience understands the dramatic irony of the speech of Oedipus and senses the tragic end ahead. And, thus their pity and fear is aroused while they also learn of the human experience.