When does foreshadowing occur and what role does it play in Oedipus Rex?

The most explicit foreshadowing is when Tiresias answers the whole mystery in the very beginning. He tells Oedipus the truth when Oedipus pushes him. The king writes it off as a trick engineered by Creon, but it encourages him to pursue the truth every more strongly, thus bringing about his own tragic end. And it's worth considering that lots of lines would work as foreshadowing for the play's intended audience, since the Greek audience would have known this story already. So when, early in the play, Oedipus says whoever committed the crime will be punished and banished, this is dramatic irony and foreshadowing for the Greek audience since they would know he was talking about himself.

Tiresias tells Oedipus the truth. Oedipus plays it off as something that was planned by Creon but it causes him to want to tell the truth about every single thing but just better every time, which brings us to the horrible end.