What is the significance of moments when Oedipus does not see the truth in front of him?
Purely and simply, it's irony. Sophocles was a big fan of rather painful, uncomfortable dramatic irony, and his original audience would have known the story of Oedipus from the myth. So from the very first moment he stepped out on stage, the audience would have known that he had already killed his father and married his mother. They would have known that he would end by going into his palace, taking the brooches from Jocasta's robe and blinding himself.
Blinding is a key theme of Oedipus Rex, because, although Oedipus only become literally blind at the very end of the play, Oedipus is metaphorically blind (unable to see what is right infront of him, unable to have any insight [in-sight] into his situation) throughout.
So everytime someone mentions sight - or Oedipus not seeing - it becomes painfully ironic:
I say that you secretly have lived most foully
with those who should be most dear, nor do you see to what extent of evil you have come.
You, even though you see clearly, do not see the scope of your evil, nor where you live, nor with whom you dwell.
As Tiresias is blind but has power to see the future, so will Oedipus only really see the truth when he is blinded. Blindness, just like in King Lear, brings with it a painful, world-worn sense of wisdom. Blindness brings in-sight.
Hope it helps!