the most unlikely hero

The words "tragic hero" accurately describe Oedipus. In fact, I can't think of a better word to describe his life than "tragic". Let's just be honest: Oedipus' life stinks (not stinks as in smells, stinks as in is terrible). When the oracle tells Oedipus he's doomed to kill his father and marry his mother, he runs away from Corinth to protect his father and mother because he loves them. Little does he know, the king and queen of Corinth aren't really his parents, and instead of running from his terrible fate, he's running to it.
When he's on the road, he kills his real father, King Laos, because he forces Oedipus rudely off the road.
When Oedipus arrives in Thebes he breaks the curse the sphinx had on the kingdom and brings Oedipus one step closer to his horrible fate. Then he unknowingly marries his mother. Then Thebes is plunged into a horrible season of famine and it's all because of Oedipus.

As far as we know, every action Oedipus made was with the best intentions. Oedipus wants what is best for his life and his people. When he hears that King Laos' murderer is the cause of Thebes' misfortunes, he vows to avenge the late king. When he hears he could be the murderer, he is hurt and angry, but still wants to keep his word.

Oedipus loves his people and justice, just as a hero ought to. He wants to do what is best for himself and for everyone. He left Corinth to protect his "parents" and solved the Sphinx's riddle to help the people of Thebes. In everything he has good intentions, but he is bound to an unfortunate fate; a tragic fate.
Oedipus Rex is a tragic play of a tragic hero because Oedipus exhibits many heroic qualities, but they lead to his demise. He doesn't get a chance to actually be heroic. It is tragic because, no matter what, Oedipus gives of himself when he cannot fix the darkness and disappointment swirling around him.

I feel sorry for him. He cares so much, and really does long for justice and freedom, but can only attain it at the cost of his life. It's tragic that someone with such heroic potential has no option but to die.