Oedipus is initially dismissive of the rumors, but as he delves deeper into his own history, he begins to realize that there may be some truth to them. This realization is a major turning point in the play, as it forces Oedipus to confront the possibility that he has unknowingly committed terrible crimes.
The Chorus's revelation of the old rumors is also significant in that it sets in motion the events that will ultimately lead to Oedipus's downfall. As Oedipus investigates his past, he uncovers a web of deceit and betrayal that ultimately exposes his true identity and brings about his tragic end.
In conclusion, the Chorus's revelation of the old rumors has a profound effect on the plot of Oedipus by setting him on a path to discover the truth about his past and ultimately leading to his downfall.