I think that dramatic irony happens at several points in the play. When Tiresias' words of "How terrible is wisdom when it brings no profit to the wise" are spoken, and the reader sees Oedipus' dismissive actions and demeanor towards such a warning, it is a moment of dramatic irony. The reader understands that there is a loaded meaning, a specific connotation in the words, but the character (Oedipus) does not. The same experience can be seen in the articulation of Oedipus' fate that he will kill his father and marry his mother. The reader is convinced or fully grasps something that the character (Again, Oedipus) does not.