Oedipus does not take the throne by force. Jocasta, the Queen of Thebes is a widow. (No one at this time—including Oedipus—realizes that Oedipus has killed Laius).
The people of Thebes have honored Oedipus. The Sphinx had terrorized them for a long time: merchants and travelers were unable to pass by the creature on their journeys unless they could answer the monster's riddle. When Oedipus does so, he makes it safe for people to travel and trade in safety once more.
He is praised by the people of Thebes; without a king, they want Oedipus to marry Jocasta and be their king. In light of all that has happened, I believe that Oedipus would have had the right to the throne in any event: what makes his position untenable (impossible to defend) is that he has married his mother, committing incest. Oedipus is a noble man and realizes the horror of what he has done, thereby punishing himself and leaving Thebes, passing the throne to his uncle Creon.