At the beginning of the play, Oedipus listens as citizens complain of a problem.

What is this problem?

I'm assuming you're talking about the play "Oedipus the King" since Oedipus is such an uncommon name. : P

At the beginning of the play the people and the priests are begging him to stop the plague that is killing hundreds of people in the city. The priests talk to him on behalf of the people and ask him to somehow lift the plague because otherwise the city will be destroyed because so many people are dying. So, Oedipus sends his brother in law, Creon, to an oracle to try to find out what to do to stop the plague.