The chorus is an integral part of a Greek tragedy. It helps form the structure of the play. It enters after the Prologue, appears at all climactic turns of plot and concludes the play with an exit song.

The primary role of the chorus is to act as a moderator or commentator. It generally occupies the orchestral space between the stage and the audience, forming a physical link between the characters on stage and the audience. The Chorus helps move the storyline forward by introducing new characters and commenting on ongoing activities and events. Through its observations, the audience gets a new perspective and sees a fuller picture of the action on stage.

The Chorus comments on and evaluates incidents, characters and themes and helps mold the audience's reactions. The Chorus also prepares the audience for what might happen next by its forebodings which may or may not come true. This adds to the dramatic tension of the plot.

The Chorus is also used to convey the vision of the dramatist to the audience. Its songs, or odes, can range from prayers and lamentations to songs of joy and grief. Sophocles has used the Chorus to magnificent effect in Oedipus Rex. The Chorus in the play consists of twelve to fifteen elders of Thebes, who react to events as they unfold on stage. The Chorus is sometimes fearful, sometimes full of praise or advice for Oedipus, and its songs refer to a variety of moods and themes ranging from tyranny to blasphemy. 

There are five Choral odes in Oedipus Rex. Each comes at a dramatic point in the play and helps the audience interpret events. It also either articulates or answers questions that arise in the mind of the audience and modulates their response.

The first Choral ode is sung just after Oedipus declares his resolve to find Laius's murderer. The ode has two parts. First that the Chorus is fearful after hearing the message from Delphi and the other lamenting the miseries faced by Thebes- 

"Countlessly, the city dies, No compassion's found.

   A deadly generation lies

   unpitied on the ground.

   Wives and mothers, young and old,

   lament by altar's shore

   pains and sufferings untold.

   Their mournful voices soar." (Page 14, Oedipus Rex, Sophocles)

By describing in horrific detail the sickness and death that has befallen Thebes, the Chorus emphasizes how high the stakes are for Oedipus, their king.

The second Choral ode comes just after the bitter quarrel between Oedipus and Teiresias, who accuses him of Laius' murder. This dramatic ode again has two parts, one, where the Chorus condemn the murderer and warn him to flee Thebes. This song further contributes to the atmosphere of terror and foreboding. In the second part, the Chorus is uncertain of Oedipus' s motives and yet is not sure whether or not to believe Teiresias

"Though Zeus and Phoebus know and see

   everything that mortals do,

   to say the priest knows more than me

   is not entirely true." (Page 32,  Oedipus Rex, Sophocles)

This conflict in its mind is transmuted to the audience as well. But the Chorus remains steadfast in its loyalty to its ideal king, Oedipus.

The third Choral ode is a moral and religious sermon, directed both at the characters on stage and the audience. The Chorus has full faith in the laws of the gods

   "May fate find me accompanied

   by purity in word and deed

   revering laws that walk on high,

   the children of the azure sky." (Page54, Oedipus Rex, Sophocles)

 It stands for the sanctity of divine laws and condemns pride and arrogance. The Chorus also deplores Oedipus’s and Jocasta's irreverence towards the Oracles. It glorifies self-restraint and piousness and is fearful for the tyrant who is proud and arrogant

The fourth ode comes, when, after Jocasta's departure, Oedipus is facing an identity crisis. This Choral song is an expression of loyalty towards Oedipus. The Chorus praises him to the point of deification and expresses gratitude to the gods that protect Oedipus. The Chorus celebrate his divine birth, ironically, just before the truth of his birth is revealed by the shepherd. 

"Who was your mother, child, disclose.

  A sprite hill-ranging Pan come near?

  .....found a son

  among his bright-eyed nymphs at play." (Page 68, Oedipus Rex, Sophocles)

The fifth and final Choral ode comments on how short-lived human happiness is. It reflects on the role of fate in Oedipus's downfall, his past triumphs, and present misery

   "If Oedipus' s fate the test,

   no human state is truly blest." (Page 75,Oedipus Rex, Sophocles)

The Chorus molds the mood and perspective of the play by its comments. It also conveys Sophocles's theme of fatalism by surrendering before the forces of fate and the gods as it laments how even the most powerful of men are ruined by fate.