It’s important to know Sophocles didn’t make the whole Oedipus story up. The myths had been around, so Sophocles’s audience would have been familiar with the tragic ending before the play began. This has a distinct impact on the tone of the plays. The actions of the characters take on a sense of irony and foreboding in this context.
Because the play doesn't have a narrator, the tone is profoundly shaped by the commentary of the Chorus. The Chorus expresses genuine sympathy for the situations of the characters, yet at the same time is acutely aware of the upcoming events.