The truth, for Oedipus, is that the truth is inescapable. At least this is one way to read the play and to understand the lesson learned by the protagonist.
His goal all along had been to escape the prophecy predicting patricide and incest. In the end, he discovers that no amount of running, cunning, or will can help him to evade the truth of the prophecy.
If freedom is understanding who, exactly, is to blame for his own demise, his children's appalling heritage, his incestuous marriage, his patricide, his city's anguish and death, and his overwhelming arrogance in trying to circumvent the will of the gods (his fate), then the truth has set him free. He may prefer ignorance, but his own arrogance ensured the truth would be told.
Truth for Oedipus sets him free from ignorance, for he did not know he had killed his father and married his mother despite his efforts to thwart the prophecy. However, the truth does not set him free from pain because he suffers great emotional pain when he discovers what he has done. Furthermore, his wife/mother hangs herself. Then Oedipus gouges out his eyes and is separated from his children, thus experiencing even more pain. Is he free? The question remains, "Free of what?"