The theme of 'Oedipus Rex' is incest, which is a six-letter word. Incest is defined as sexual relations between two people who are closely related, such as siblings or parent and child. In the play, Oedipus unknowingly marries his mother, Jocasta, and has children with her, making it an example of incestuous behavior. This theme of incest serves to illustrate the consequences of hubris and fate in the play.