What I find to be the most powerful irony of the play is that which makes it a tragedy as well: what makes Oedipus great--his pride and pursuit of the "truth"--also results in his tragic fall--his discovery that he murdered his father. This irony also develops through the metaphor of sight: Oedipus wants to "see" the truth, but then blinds himself because of what he discovers, what he finally "sees."
There is extensive irony in the myth and play of Oedipus. The most fundamental comes in the situation of Oedipus himself. The prophecies said that he would kill his father and marry his mother. Horrified, his parents left him on a hillside. He was found and raised by others, and did not know his parents. Because of this, he later...killed his father and married his mother.
In addition to this overarching situation irony, there are several others. Oedipus asks Teiresias for help figuring out who did the crime that brought the curse on his city...but it was Oedipus. The very man who set out to cure his city is the one who tainted it. That's great irony.