At times, Jocasta and Oedipus rely upon the predictions of the oracles. At other times, Jocasta and Oedipus condemn oracles and even mock the gods. Why is it understandable that they would at first trust the oracle and the gods?
In the beginning of the play, Iocasta and Oedipus trust the oracles presented by the Gods because they are integral to their belief systems and culture (this would have been customary during the time period). Iocasta and Oedipus both believe that the Gods have actually determined their fate and in turn are responsible for their well-being and happiness.
Moreover, it is clear that both Iocasta and Oedipus fear the prophecies set forth by the oracles and behave in particular ways in response to these prophecies. For example, both Iocasta and Oedipus behave hastily in order to avoid the prophecies that they've discovered from coming true. Iocasta orders the murder of her son when he is less than a week old to save herself and her husband and Oedipus flees Corinth in order to save his parents from himself. It is in attempting to avoid fate in these ways that Iocasta and Oedipus both ironically seal their fates.
At times, throughout the play, Iocasta and Oedipus both mock the oracles and call question to their truth. First, Oedipus doubts the ability of Teiresias and calls into question his ability to know the truth of the situation. Then, Iocasta calls into question the validity of oracles when she states that the prophecy given to Laius was never fulfilled. She notes that their son, having been murdered, could not have slain Laius as the prophecy proclaimed he would. Both of these instances demonstrate the willingness of the characters to overlook the truth of the oracles.
When one considers the motivations that Oedipus and Iocasta have for rejecting the truth of the oracles it is clear that these denials serve to allow them to continue in their ignorance throughout the course of the play. By denying the power of the oracle and the truth of the prophecies, neither character has to come to terms with the true tragedy of their fate.