This is a hard question, because the idea of forgiveness is not really something that is embedded in the Greek culture. Forgiveness is more of a Christian concept. Hence, to put that idea back into the time of Sophocles is sort of anachronistic. However, with that said, there are few observations that might help in thinking about the issues at hand, which lead to his "forgiveness."
First, even though Oedipus was filled with hubris and pride and these qualities blinded him, he acted unwittingly. He did not intentionally marry his mother and kill his father. The ancients did distinguish between intentional and unintentional transgressions against the gods.
Second, when we introduce the idea of fate, things get complicated. In other words, if Oedipus was fated to kill his father and marry his mother, then his responsibility is mitgated at least a little. These two points lead to the fact that he should forgive himself.