In determining the themes of Sophocles's Oedipus Rex, it's important to understand the distinction between a theme and a motif.
A theme is a dominant central idea, message, or moral that the playwright wishes to convey to the audience. A theme is often a universal truth, at least at the time the play was written, and is often centered around society, human nature, or the human condition.
A motif is a recurring idea, image, symbol, or event that supports, reinforces, or helps to explain a theme.
It's also important to understand the context in which the plays were written and performed. In Ancient Greece, plays weren't intended for entertainment—they were written and performed in order to teach the audience a moral lesson.
Most of the plots of Greek tragedies were based on ancient myths and legends that the audience grew up reading and hearing. The audience already knew how the story was going to end when they went to see a play, so what was interesting to the audience was not the myth or legend itself, but how the playwright dramatized the myth and how the playwright dramatized the lessons to be learned from the myth.
A good question to ask is whether a particular idea or event is a lesson or moral to be learned (a theme) or if the idea or event simply supports the lesson to be learned (a motif).
A major theme in Oedipus Rex is that the gods, not individual human beings, determine an individual's fate—the idea of fate versus free will.
No matter what Oedipus or anybody else does, no matter how much free will Oedipus exercises (or thinks he does), he can't escape his fate. He will kill his father and marry his mother because the gods said so.
The Oracle is a major motif that supports the theme of fate versus free will. The oracle made predictions based on the will of the gods and provided guidance, often in cryptic terms. The oracle is discussed or consulted several times during the play. Nothing having to do with consulting the oracle is a theme of Oedipus Rex, but the oracle is a motif that recurs throughout the play and which supports the theme of the will of the gods versus free will.
Another major theme of Oedipus Rex is that excessive pride (hubris) caused Oedipus' tragic downfall. There's a lesson to be learned from the choices that Oedipus makes, particularly the mistakes that he makes, based on his pride.
A motif that supports this theme is blindness. In Oedipus Rex, blindness symbolizes ignorance in the sense that some characters simply don't know that they're acting against their own or another character's best interest. Blindness also represents a prideful refusal to see and acknowledge the truth of the prophecies, as well as a refusal to recognize one's own faults.
In Oedipus Rex, Oedipus's pride and his refusal to recognize the will of the gods lead to his tragic downfall.