What is the atmosphere of Oedipus Rex? How is Tiresias significant to the atmosphere?
Another word I would use to characterize the emotional atmosphere of Oedipus Rex is tense. First, audiences at the time would already have known the story of Oedipus, so dramatic irony exists even at the play's outset: the audience knows more than the characters themselves do, which creates tension. Further, tension runs high as people in Thebes suffer and the crops fail; suppliants come to their king, Oedipus, to beg his help, and he assures them that he feels their suffering even more than each of them individually because he feels it for the general welfare too. Oedipus almost seems indignant that the people think he has not already acted on their behalf; in truth, he has sent Creon to the oracle to ask for some answers.
While Creon is gone, Tiresias comes at Oedipus's request, and his vague and evasive responses to Oedipus's questions enrage the king. Tiresias assures him again and again that he does not want to know the truth, that it is better for Oedipus not to know what Tiresias knows. Oedipus insults the prophet as well as the god who has given Tiresias the gift of prophecy, even poking fun at the old man's blindness. This increases the tension dramatically, especially since the audience knows that Tiresias is right: Oedipus really does not want to know the truth (and it is never a good idea to criticize the gods).
Tension increases even more as Oedipus interacts with Creon, accusing Creon of plotting against him. His conversations with Jocasta, his wife (and mother), also increase tension as she begs Oedipus to give up his quest for the truth and he accuses her of being elitist and afraid that he descends from the lower orders rather than a king. The tension continues to grow and grow until Oedipus finally puts all the pieces together, realizing that all of his accusations of others are baseless and that he is the one responsible for all the trouble in Thebes.
The setting of Oedipus Rex is one of tragedy. The city of Thebes is suffering from a horrible plague and people are in great pain. In this context, their leader, Oedipus is trying to solve this problem and bring comfort but it is to no avail. From this perspective, we can say that the situation is also very tense.
In this context, Tiresias is important, because he is the wise seer that knows what is going on and more importantly he knows why it is going on. The irony of the situation is that Tiresias is blind. The blind seer has supernatural insight. What makes this situation even more pronounced is that Oedipus is physically able to see perfectly, but when it comes to deeper matters like his identity he is really the one who is blind.
In a confrontation between Tiresias and Oedipus, Tiresias tells Oedipus point blank that he is the murderer that he is looking for. In a word, Oedipus is responsible for the plague of Thebes. In time, Oedipus is able to see this truth.