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.
Arguably, that what Sophocles is talking about that fate is not up to the person, it is the divine will of the God's.
The Ancient Greeks did not believe that fate was determined by the person, rather it was up to the God's. That is why no matter what steps Oedipus took, he was doomed to make the same mistake. Whether he knew it or not, even after persuing his investigation into the death of Laius, he had still married his mother, killed his father and bore children, so it could be argued that the discovery was always going to happen.
Fate is the divine will of the gods, and to believe otherwise in the Ancient world was considered hubris and an insult to the Gods. Which is why Euripedes was so controversial, but I shall leave that for another day.
It won't let me reference books so:
Antigone; Oedipus the King; Electra; (translated by H.D.F. Kitto; edited with an introduction and notes by Edith Hall. Oxford : Oxford University Press, 1994