In Oedipus Rex, how does Oedipus struggle with free will?

Through most of the play, as far as Oedipus is concerned,
he isn't "struggling" with free will at all, he is acting upon it.  However, once the
real truth is unraveled, his free will and past actions based upon his own choices are
revealed as his tragic flaw -- the hubris of believing that
he could outsmart the gods and avoid the prophecy that he would "kill his father and
marry his mother."

Oedipus Rex is a
Tragedy in the true classical sense of the word, which means that Oedipus must be the
architect of his own demise, not a victim of circumstance.  This sets him up for a
struggle with the pre-ordained oracle from the gods, suggesting that Oedipus, if we
observe his actions as representative of the human struggle, is destined for his
downfall because he attempts to act upon his own impulses (or free will), rather than
acting in service to the gods.

So, it is Oedipus' false
sense of free will that sets him up for tragic consequences in the following
ways:

• He leaves the home of his assumed father,
  Polybus, when he learns that there is a prophecy that he will kill his father and marry
  his mother. He hopes to avoid this outcome by this action, not knowing that Polybus is
  his adopted father.


• While out on the
  road, he meets a man who is just as stubborn as himself and refuses to move and allow
  Oedipus to pass.  In a rage, he kills this mysterious man (so like himself) -- the man
  later revealed to be his natural father,
  Laius.


• He next solves the riddle of the Sphinx and wins
  the hand of the widowed Queen of Thebes, Jocasta, and they marry.  Later, he discovers
  that Jocasta was Laius' widow.


• And he finally learns the
  truth of his adoption, making the connection that Jocasta is, in fact, his
  mother.

Ultimately, it is Oedipus' attempt to
rely upon his free will, rather than his struggle against it, that seals his doom.  This
Greek Tragedy follows the standard party line:  No human can escape his fate as it is
predicted by the gods, and woe to him that might attempt to do so!  The force that
Oedipus struggles against throughout the play is the will of the gods as made manifest
through the prophecy about his future.