The downfall of Oeidpus the King was not only caused by the Gods through his fate but also by his own actions. Oeidpus" fate was known by his birth parents and that's how this tragedy began.
When Oeidpus was a grown man he tried to change what the Gods had in store for him but little did he know his own actions were bringing him towards his fate. After Oeidpus had married Iocaste, not knowing she was his birth mother and solving the riddle of the Sphinx this began to change. More questions arose about the death of Laios and of who was his band of murders. At this time Oeidpus was unaware of who he had slayed that day on his way from Cornith to Thebes. As the time pasted he began to realize that it was King Laios and if anyone found out the truth he would be denounced as King and self exiled from Thebes. At this time Oeidpus still was unaware that Polybos and Meroope were not his biological parents and he figured by coming to Thebes and being away from them would change his fate.
As time went on things became clearer to Oeidpus and finally the day came when the messenger has come to him with horrible news that Polybos was dead, Oeidpus believed he had changed his fate. As the messenger went on and confessed to him they King Laios and Iocaste were his biological parents he became ill. Oeidpus realized even by leaving Cornith, starting a new lid having a family he was enduring his fate. His own choices had failed him and brought him to the life he was running from.
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I believe that the Gods and Oeidpus" own choices lead to his downfall because even though Apollo told Oeidpus his fate, his own actions caused his fate to take place unknowingly. If only Oeidpus new Polybos and Merope were not his biological parents and had stayed in Cornith he would not have endured all the pain and misery of killing his father, marrying his mother and having children with her. Oeidpus finally realized that there was no way for him to prevent what the Gods had in store for him.