Who is the Choragos and what is his task in scene 1?
The Choragos is the leader of the chorus, and he is also the member of the chorus who participates in the drama of the play as a member of the cast. In his role, he interacts with people in the play. In scene I, his task is to encourage Oedipus to summon Teiresias, the blind prophet, to figure out who killed the king. The Choragos first urges Oedipus to let Phoebus, or Apollo, figure out who killed the king, but Oedipus responds that mortals cannot force the gods to do what mortals want. Then the Choragos says, "I know that what the Lord Teiresias sees, is most often what the Lord Apollo sees. If you should inquire of this from him you might find out most clearly" (lines 304-307). In other words, since Teiresias sees what the gods do, Oedipus should call on him to find out who killed the king. In addition, the Choragos tells Oedipus that the king was killed by wayfarers.
The Choragos is the "leader" of the Chorus. He does most of the talking in the Chorus. The rest of the Chorus will chant or repeat what he says, but he is clearly the leader.
In ancient Greek tragedy, the Choragos might very well have been the leading man, even top billing over Oedipus, as the Choragos was an actor who would have commanded respect. The Choragos would have occupied the highest level on the stage and been on stage nearly the entire play.
In Sophocles' play Oedipus Rex, the Choragos is the voice of reason. He introduces Tiresias and urges Oedipus to listen to him. He also urges other characters to avoid extremes, to practice moderation, and see and hear the truth.