As in many of our surviving Greek tragedies, the chorus, generally speaking, represents the viewpoint of someone who is not a king, queen, prince, princess, or other upper-class person.
In Sophocles' Oedipus the King, the chorus is given the persona of elderly men from Thebes. In their initial conversation with Oedipus, they advise him to consult Teiresias about the murder of Laius (and Oedipus follows their advice). They also provide Oedipus with any information they know about Laius' murder. So, in their opening encounter with Oedipus, they serve as advisors and informants.
Later in the play, after Oedipus argues with Creon, the elderly Thebans again take on the role of advisor as they urge Oedipus not to suspect Creon of being disloyal to him.
After Oedipus blinds himself, the chorus express both pity and horror at Oedipus. They actually think that Oedipus would have been better off if he had killed himself.
So, in Oedipus the King, the chorus' role with respect to Oedipus himself appears to be that of advisor, provider of information, and to serve as a group of citizens who reacts to what he has done after learning of his true identity. These roles are not dissimilar to the roles taken on by other choruses in Greek tragedies.