It's important to remember that although the events in Oedipus Rex that lead to Oedipus's tragic downfall are primarily the result of his own tragic flaw (hamartia) of excessive pride (hubris), the events that brought Oedipus to Thebes and into his current situation are not entirely the result of his own hubris-driven decisions.
As the story of Oedipus's life is revealed piece by piece in the play, Sophocles shows us his "tragic vision" for Oedipus Rex—that Oedipus is a victim of many circumstances and events throughout his life that were beyond his control, which contributed to his downfall, along with his own hubris.
Oedipus was a baby when he was saved from death in the mountains by a herdsman and given to King Polybus and Queen Merope of Corinth to raise as their own child.
Oedipus has no idea that Polybus and Merope aren't his birth parents until that fact is revealed by the messenger in the play.
This is the moment in the play called the peripeteia , part of Sophocles's overall "tragic vision"...
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