In "Oedipus the King," does pride or fate bring Oedipus to his downfall, and could he have done anything different to have a happy ending?
I believe that Oedipus' downfall is dictated by fate. I do not believe that his pride is at fault, and I do not belief he could have done things differently to exact a better outcome.
In terms of the question of Oedipus' fate, a prediction was made about what would happen to Oedipus, so if one is to believe in predictions, he had no control. However, it does not stop his parents from trying. He is abandoned by his parents on a hillside to starve with the hope of thwarting the prophecy. Oedipus is saved and raised in another royal household. Hearing of he prophecy, he leaves his adopted home to protect his "father" (not knowing he is adopted), only to meet his real father on his way and kill him.
A terrible monster called the Sphinx kills all passing travelers who cannot answer his riddle. Oedipus does this, and defeat the Sphinx so that travelers may safely move about. Oedipus is brought as a hero into the city of Thebes (where he was born, though he does not know this). There he is given in marriage to a widow as a reward: however, the woman is his mother, and so the prophecy is fulfilled.
Sophocles and his culture (the Greeks) believed in predestination: that your life was mapped out for you before you were born, and that nothing could be done to change the fate the gods had in store for you.
In this case, Oedipus tries to do the right thing, which immediately puts him in the position of not protecting his [adoptive] father as he had hoped, but killing his real father and marrying his mother.
There was nothing Oedipus could do to provide himself with a happy ending.