Whom does the chorus speak for?
The main function of the chorus in Greek drama is to comment on the action of the play, usually from the perspective of citizens of whatever area is represented in the play.
In Oedipus Rex, the action takes place in Thebes, so the chorus stands in for the people of Thebes. As such, the chorus is not always reliable or decisive. Over the course of the play, we see the chorus move from full-fledged support of Oedipus to worrying about how he will negatively affect the fortunes of Thebes.
At some points, the chorus speaks at length directly to the audience. This usually happens at the end of an act during sections called the "strophe," and the "antistrophe." At other times the chorus may interact verbally the characters themselves. Think of this as the characters talking to a committee of local citizens.
It is important to remember that the chorus is not infallible and can make errors in judgment. In fact, part of the development of a Greek play may include changes on the part of the chorus' perspective and/or attitude.