A reminder: one question per day, please.
The chorus in Oedipus Rex is vital to the religious aspect of the play that honors Dionysus. It has several functions within and outside the play.
Its primary role is to be the ideal audience. The chorus shows the audience literally how to act, how to be involved in the ceremony of the play, how to uncover the mystery and catharsis of the tragedy.
The secondary role is to comment on the action of the play. In this way, the chorus in an intermediary between the actors and the audience. The chorus adds an objective commentary on the plight of Oedipus as he battles with Creon, Teireisias, Jocasta, his witnesses, and himself. Even though they comment, they are complacent: they do not dictate or sway the action of the pay.
Thirdly, the Chorus urges each character in the play to not succumb to hubris and human weakness; instead, they urge moderation and reason. This moral dimension is crucial to the religious festival itself.