The biggest irony in "Oedipus the King" is that although Oedipus spends his entire life trying to avoid the prophecy that he would kill his father and marry his mother, he still completes both actions without knowing it. There are other examples of irony in the famous play, but this is the most important one.
There are many examples of irony in "Oedipus the King." The most predominant example is Oedipus' attempt to avoid the prophecy that has been placed upon him. After an oracle tells Oedipus that he is destined to kill his father and marry his mother, Oedipus leaves his home in Corinth to avoid his parents and make sure this prophecy does not come true. However, Oedipus does not know that the people he believes are his parents are not actually his parents. The irony comes when Oedipus kills a man he meets on his travels, who happens to be his real father, and is later offered the hand of Queen Jocasta in marriage, who happens to be his real mother. In an effort to avoid the people he thought were his parents, he runs straight into the two people who happen to be his true family.
Another example of irony in "Oedipus the King" comes when King Oedipus attempts to discover why the kingdom of Thebes is cursed without realizing that he has brought the curse on the town by marrying his mother Queen Jocasta. A third example takes place before the play begins. Oedipus' real parents attempt to thwart the prophecy by putting their infant out to die of exposure only to have Oedipus saved by a passing shepherd. These are only a few of the many examples of irony in "Oedipus the King."