That is a great question and one that is not easy to answer. In fact, in the end, there will be no one correct answer, because Sophocles in particular and the Greek tragedians in general loved to problematize. They would take two important or core beliefs and put them against one another. For example, in your case, Sophocles pit human freedom and the will of the gods against each other. So, on the one hand there are these prophecies that must come truth (Oedipus will marry his mother and kill his father). On the other hand, Sophocles has Oedipus drive the narrative of the play by making decision and having the feeling that he can solve anything. For these two reasons, he is considered tragic. He choses his own fate, but at the same time his fate is also chosen for him. In the end both are true and we need to be comfortable with that.
It seems that Sophocles depicts human freedom, at least Oedipus', as an element that faces some level of constriction. On one level, Oedipus strives to be a good ruler and govern with the public interest in mind. He promises his people that he will investigate and find the cure for the plague which has fallen upon his people. At this point, his commitment to his people has curtailed his own freedom because he is unable to extract himself from the promises made to his people. The statement here is that if individuals are in the role of public policy, they lose some level of freedom. Another constriction on Oedipus' freedom seems to be the forces of fate. Sophocles seems to be making a larger statement on the role of individual freedom in light of fate. The issue of fate vs. free will is a critical one in the play, and one that must be seen in the light of freedom, as Oedipus seeks to exert the latter over the former. The author's declaration of his futility through the actions in the play reveals Sophocles' perception that human freedom is limited in light of external elements.