There is a definite sense in which the Chorus of this play changes in terms of what it talks about and the tone of its speech. At their first entrance they mourn what is happening in Thebes and the many deaths that the city has experienced. They appeal to the gods to end this plague and to restore them. They clearly fulfill their role as reflecting the action of the play. However, as the play goes on, it is clear that their role develops into criticising the characters, especially Jocasta, as she tries to convince her husband to ignore the various omens and prophecies he has received.
Finally, at the very end of the play, Sophocles gives the Chorus the job of expressing the moral or theme of the play in the last lines as the tragic fate of Oedipus is reflected on and the mood of the Chorus becomes very bleak and depressing as they face the enormity of what has happened to Thebes by focusing on what happened to Oedipus:
Now what a black sea of terror has overwhelmed him.
Now as we keep our watch and wait the final day,
count no man happy till he dies, free of pain at last.
The Chorus therefore seems to capture the mood of the play as the action progresses, finally ending up with the very serious and sombre message that Sophocles wishes to impart to his audience through the tragic example of Oedipus.