"The title tyrannos has then a magnificent ironic function; but if it makes a great contribution to the complexity of the play's texture, it raises some problems as well. For the word meant more to the fifth-century audience than a usurper who replaced the hereditary king: the tyrannos was an adventurer who, however brilliant and prosperous his regime, had gained power by violence and maintained it by violence.
The connection of the tyrannos with violence is forced on the spectator when the chorus sings, "Violence and pride (hybris) engender the tyrannos." The succeeding sentences of this choral ode are an estimate of the origin, nature, and end of the tyrannos in terms of the current moral and political tradition of the late fifth century.
What is the reason for the chorus' attack on Oedipus? And why does it take this particular form? According to Jebb "the strain of warning rebuke" is suggested by "the tone of Oedipus towards Creon"; but this does not seem an adequate explanation. The chorus' last word on the subject of Creon was a declaration of complete loyalty to Oedipus. "I should be clearly insane, incapable of intelligence, if I turned my back on you." (690-1). The change from this to "Violence and pride engender the tyrannos" is clearly a decisive change: it must be due to something that has happened since the quarrel with Creon.
Not much has happened, but much has been revealed. Oedipus came to Thebes with blood on his hands, and one of the men he had just killed was a person of some importance, for he rode in a carriage and was accompanied by a herald. True, Oedipus struck in self-defense, but none the less the chorus has come to know an Oedipus they had not suspected, a man of violence, who can say, with almost a touch of pride, "I killed the whole lot of them." (Kteino de tous xumpantas)
Hubris phyteuei tyrannon. The elevation of Oedipus to the throne of Thebes was preceded by the bloody slaughter on the highway.
But this is not all. Oedipus has good reason to suspect that the man in the carriage was Laius, the hereditary king of Thebes, and the chorus is afraid he might be right. If he is, then Oedipus won his power by killing the hereditary king and taking his place both on his throne and in his marriage bed - like Gyges of Lydia, one of the classic types of the tyrannos; Gyges is in fact the first man to whom the title is applied in extant Greek literature. Hubris phyteuei tyrannon-violence engenders him, for it is the instrument of his accession to power."
Why Is Oedipus Called Tyrannos?
Bernard M. W. Knox, The Classical Journal Vol. 50, No. 3 (Dec., 1954)