Oedipus Rex has a grave, ominous, and harsh tone. The play privileges the supremacy of fate and the divine through Oedipus's inability to escape the prophecy as well as the chorus's frequent singing of the power of the gods and goddesses. At the same time, Oedipus is portrayed as ignorant, as the reader becomes aware of Oedipus's fate before Oedipus himself understands what his future holds. Oedipus Rex, then, directs a disapproving tone at any who fail to heed the words of the gods.
Simultaneously, it is by trying to avoid the prophecy that Oedipus ends up fulfilling it. As such, there is an overarching sense of inevitability, with Oedipus's cruel fate in many ways appearing unavoidable. There is a tone of futility, both in regards to human suffering—which can only be prevented by appeasing the fickle gods—and in regards to Oedipus himself.
Sophocles's tone is ultimately harsh, a combination of the dark subject matter, the formal style, and his condemnation of Oedipus. This tone, however, adds weight to the climax of the play. Jocasta's suicide and Oedipus's subsequent gouging out of his eyes feel so impactful and appropriate to the audience precisely because they match the overall tone of the play.