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Apparently, thanks to Wikipedia, there are three operas with that name:
"Rex" is Latin for "King". Oedipus Rex means "Oedipus the King".
Thebes is the setting of Oedipus Rex because it is the place where the story begins.
'King' is an English equivalent of 'Rex' in the play 'Oedipus Rex'.
The shepherd in Oedipus Rex is the person who rescues Oedipus Rex as a child. The shepherd also confirms the main character's fate.