In Aristotle's Poetics, written in the century after the appearance of Sophocles' Oedipus the King, the author examines genres such as tragedy, comedy, and epic poetry. The surviving text of the Poetics makes it clear that Aristotle regarded Sophocles' Oedipus the King as a tragedy in its finest form.
In Poetics Chapter 13, for example, Aristotle says that tragedy should not focus on people who are "eminently good and just" and someone whose misfortune is a result of "some error or frailty". Aristotle goes on to list Oedipus as one of those who fits these requirements.
In the same chapter, Aristotle says that in tragedy there should be a reversal of fortune from good to bad. Again, Aristotle sites the story of Oedipus as one of those that provides the basis for "the best tragedies."
In Poetics 14, Aristotle says that tragedy ought to stir up feelings of pity and fear in the audience and he singles out Sophocles' play as a prime example of this ("from hearing the story of the Oedipus"). Later in that chapter, Aristotle again cites the story of Oedipus as an example of a tragedy in which the horrific events are "done in ignorance."