In Oedipus Rex, how is Oedipus led to commit the sin of killing his father and marrying his mother?

Oedipus is a victim or a tool of fate, a destiny that he cannot escape.

Oedipus, who is abandoned to die by his real parents, is rescued by a herdsman and given to the King and Queen of Corinth.  Growing up a Prince, Oedipus learns that the King and Queen are not his birth parents.  Seeking the help of the oracle at Delphi, he learns of the prophecy of killing his father and marrying his mother and is so horrified, that he is determined to outwit the prophecy.  He leaves Corinth.  

As he travels near Thebes, he is assaulted by an old man and his servants, he strikes back, killing the man.  Oedipus does not know that the old man is Laius, his real father.

He confronts the monster Sphinx, answers the riddle and is hailed as a champion.  Approaching the widow, Jocasta, Oedipus seeks advice about the prophecy.  She does not believe in prophecies, since the one told to her did not come true, her child died.

When he discovers that his father, the King of Corinth has died from natural causes, Jocasta tells him that it is proof that the prophecy is never going to come true. The two marry and have four children.  Oedipus has married his mother.

Events in the story unfold to make Oedipus believe that he has avoided fulfilling the prophecy, when in fact, his fate is a set path, a road he cannot avoid.