Oedipus at Colonus is a weird tragedy in that the climax of the play is not really tragic. I even hesitate to call this play a tragedy: yes, some machinations of fate are at play, and yes, Polynices' obstinacy to go to his death are sad, but the story here is of Oedipus and his fate to die in glory with the gods.
Oedipus is practically assumed as Christians would later call it—it‘s such a supernatural ending to such an earthy tradition, no?