It was believed by the Greeks that even Zeus was bound by the iron chains of fate and necessity. Fate was what made prophecy possible, because if fate did not exist then the future would be contingent rather than necessary. As Teiresias points out when he reveals the future to Oedipus:
It is not your fate
to fall because of me. It’s up to Apollo
to make that happen. He will be enough ...
Fate, however, is not so much in opposition to free will as a different perspective. From our perspective as mortals, we make choices according to our free will. But our free actions are conditioned by our own natures, and thus sub specie aeternitatis, they appear as necessary. Thus whatever choices Oedipus makes still lead him to fulfill his fate.