Free will does not exist.
At least, according to Sophocles it doesn't. Take Oedipus Rex as a perfect example, he hears an oracle predicting his future, a future so horrible that he cannot bear the thought of it. So he packs his bags and leaves his father and mother for fear of killing one and sleeping with the other. However, his fate follows him and he ends up doing exactly as predicted.
The real kicker here is that it's almost as if the gods are playing a joke on Oedipus. First they tell his birth parents that he is fated to kill one of them and then they tell his adoptive parents the same, forcing Oedipus to flee and, in turn, fulfill the prophecy on the way to Thebes. I'm sure the gods knew that this would be how Oedipus would fulfill his fate. He never stood a chance. He never had a choice.
There are a lot of different interpretations for Oedipus Rex, but I think an important aspect lies within these lines: Pride is the germ of kings;
Pride, when puffed up, vainly, with many things
Unseasonable, unfitting, mounts the wall,
Only to hurry to that fatal fall,
Where feet are vain to serve her. But the task
Propitious to the city GOD I ask
Never to take away!
GOD I will never cease to hold my stay.
Fate has to be achieved somehow, so how, exactly, did Oedipus go about achieving his? It was Oedipus's own pride that kept him from truly heeding the oracle. Yes, he believed it at first and fled from Corinth in fear, but when it was presented to him in the most plain of terms, he could not accept it. Even when it was obvious, without a shadow of a doubt, that he had been sleeping with his mother and killed his own father, he could not believe it until he had been told by one who had witnessed his fate. His pride led to his fall, much as it does for most legends and myths.
Towards the end of the play, Creon says this: Even you might trust what the God answers, now.
This line further cements my idea that it was pride that led to Oedipus fulfilling his own destiny. Also, I think that this is a hint to Oedipus that it was his own disregard for the gods that brought this upon himself. He was cursed from birth and yet still lived as a king.
The message is simple here: people should not be so conceited to believe that they can create their own fates. Your life is set in stone and there is nothing you can do to prevent what is supposed to happen from happening.
At least, according to Sophocles.