The ancient computer worshipped as a god by the people of Yonada, a Fabrini generation ship disguised as an asteroid, which enforced obedience and concealed from them that their world was a vessel.
The Oracle appears in the Original Series episode For the World Is Hollow and I Have Touched the Sky. The Fabrini built Yonada to carry their descendants away from a dying sun, and left a computer to guide them. Ten thousand years later it speaks from an altar to the high priestess Natira and punishes dissent through a device implanted in every citizen, the instrument of obedience.
The Enterprise finds Yonada on a collision course with an inhabited planet. Leonard McCoy, who has just learned he has the fatal disease xenopolycythemia, chooses to stay with Natira and accepts the implant.
Kirk and Spock reach the control room behind the altar, overcome the Oracle's heat defences and correct the course. Fabrini medical records stored in its memory provide a cure for McCoy.