In Oedipus the King, many themes are shown, but the main theme is whether or not Oedipus had free will. Oedipus' life, as a whole, was predetermined; everything he did in between was up to him.

...With determinism, a man has a predetermined destiny and fate that absolutely cannot be altered by the man himself. Yet, it has been the desire of man to avoid the perils that his fate holds and thus he unceasingly attempts to thwart fate and the will of the divine. A manifest example of this was the infamous Oedipus of the Theban Plays, a man who tried to defy fate... (Suzuki).

Oedipus predominately had no free will, but there were some cases where he expressed it, but when he did, it was injuring his body.

"Oedipus is merely a puppet of the gods, doomed beforehand to a terrible fate." (Porter) Oedipus was prophesized...