The Chorus in the play Oedipus has three basic functions. Firstly, to act as bystanders throughout the play, observing what goes on, reacting and offering opinions at regular intervals throughout the play (e.g. when Oedipus accuses Croon irrationally of conspiring against him, the Chorus says "To one who fears fall, his words seem good; O king, swift counsels are not always safe") In this role they are important to the play as they often offer the voice of reason during moments of heated debate, the voice of fear and confusion during Oedipus' downfall, which seems to mirror the audience's reaction and emotion in many situations throughout the play. The Chorus are also used as a sort of receptive audience for passages of description over what has gone on behind closed doors (e.g. after Jocasta has realised the truth the second messenger describes the scene inside the palace to the Chorus: "Within the porch, straight to the couch she rushed, her bridal bed, and tore her hair").
Secondly, the Chorus offers a sort of running narrative for the audience. This function is equally vital to the success of the play. The Chorus often speaks in this way at the end of a scene to clarify what has just gone on. (e.g. at the end of the first scene, the chorus clarifies the diseased state that the city of Thebes has descended into: "Still breeding plague, unpitied infants lie and wives and mothers, grey with hoary age by every alter mourn.").
Finally, the Chorus is used to keep the continuity during the play. In modern theatre, the plays are normally split up into scenes and acts. However, the Greek kept the continuity between these natural gaps, by having a narrative section. This prevented the necessity of having breaks in the action and also kept the audience up to date with what had just gone on, and perhaps offering some insight or other. In this role, I think the Chorus is very helpful to the audience, as it succeeds in keeping the audience 'on the edge of their seats'.