There are two instances in Oedipus in which the characters attempt to override fate. First, we have Oedipus's parents, who believe the oracle's prediction that their son will murder his father and marry his mother. They completely believe this. We know, because they send him to be left to die from exposure to avoid having it happen. Later, when Oedipus--who had been saved by a shepherd and taken to another city and raised as a prince--learned of the prophesy, he believed it, as well. He believed it so completely that he left his place as heir to the throne to distance himself from his (adoptive) parent to ensure the prophesy never came true.
Here's the problem, though: All of these characters clearly believed the prediction enough to take drastic measures to avoid it, meaning they believed it could not be avoided through human will. But if they really believed it that much, how is it possible that they thought that even drastic measures could avoid it? It is as though they believed the prophesy as much as they believed they could avoid it--but if they believed they could avoid it, then it wasn't really fate, was it? Why take such drastic measures? Particularly in Oedipus's case--why not just be very careful to not kill a man (for starters) then get married to a woman whose history you don't know?