Oedipa theorizes near the end of the novel that she has either stumbled onto the Tristero's plot, or she has been self-deceived into believing in the Tristero, or she has been deceived by a plot against her, or she is fantasizing some such plot.

Considering a passage earlier in the novel, it seems that at least her two notions of self-deception and fantasy can be marked off. Although she may exist as a typical suburban house wife who goes to Tupperware parties, gets drunk, and watches television when she's not tending to the house, she is not trying to escape her boring life by creating a conspiracy or dilemma to amuse herself because departure from this lifestyle is impossible.

"What did she desire to escape from? Such a captive maiden... soon realizes that her tower, its height and architecture, are like her ego only incidental: that what really keeps her where she is is magic, anonymous and malignant, visited upon her from outside and for no reason at all. Having no apparatus except gut fear and female cunning to examine this formless magic, to understand how it works, how to measure its field strength, count its lines of force she may fall back on superstition."

Thus, Oedipa is where she is because of chance, luck, something she has no control of. She does not choose where she is, nor can she escape it, because it's formless magic that dictates her life. Fate prevails over choice.

So, Oedipa cannot create this fantasy. It already exists and she has become part of it.