In his Poetics, Aristotle famously defines tragedy as follows:
Tragedy, then, is an imitation of an action that is serious, complete, and of a certain magnitude; in language embellished with each kind of artistic ornament, ... in the form of action, not of narrative; through pity and fear effecting the proper purgation of these emotions.
The action of Oedipus Rex is of a "certain magnitude" or "grandeur" in that it deals with the fate of cities and kings. It has a distinct unity, starting with a problem, the plague, and moving through the solution, discovery of the killer of Laius and removal of the pollution from the city, ending the plague. The action is serious, dealing with matters of life and death, and of the relationship of the city to its gods, and, in the case of the individual Oedipus, the working out of the curse on his house. It is also unified in place, time, and character.