I love this question! It really helps you to think creatively about the various characters in the play and their function. I would pick Teiresias and the object I would pick would be a mirror. Let me explain: in the play, as a prophet, he is called upon to explain to Oedipus the reason for the plague that is causing so much trouble in Thebes and to point a finger at who is responsible. In response to his reluctance to reveal his information, Oedipus goads him, insults him and accuses him of betraying him. Yet, in a highly significant dialogue, Teiresias again and again confronts Oedipus with the truth in Scene 1, and yet Oedipus is blind to it or does not want to accept the truth of Teiresias's words. Consider the following words from Teiresias:
I say you live in hideous shame with those
Most dear to you. You cannot see the evil.
You are the madman. There is no one here
Who will not curse you soon, as you curse me.
But I say that you, with both your eyes, are blind:
You cannot see the wretchedness of your life,
Nor in whose house you live, no, nor with whom.
Note how in each of these exchanges Teiresias takes what is thrown at him by a desperate Oedipus and turns it back on to him, confronting him with the unyielding truth just as a mirror confronts us with our own reflection which cannot be ignored or denied. The tragedy of this play lies in the fact that Oedipus is only able to accept the reality of these "reflections" at the end.