One of the key concepts and themes within Oedipus Rex is the concept of fate (the divine will of the Gods) and that of the choices and actions of individuals. Clearly the prophecies that oracles make in the play is a central focus throughout the play. Yet equally we can see that man's ability to change his future through his own choices and decisions is explored as well. Interesting question when you think about it - do you think your future is already mapped out for you or do you think there are an infinite number of futures which are dependent on decisions that you make?
Of course, in Oedipus Rex, we see the triumph of fate over free-will. In spite of all he does to be a good king and to protect his throne, eventually he is forced to concede that the power of fate is stronger than the power of free will.