Om is a god of Discworld. He features heavily in Small Gods, where he has manifested himself as a golden bull, but due to a cock-up on the believers front, landed up as a tortoise instead.

This annoyed him.

It actually took him several years to figure out that he wasn't a tortoise and was, in fact, a god. Then there was all the dangers of the eagles¹ and finally he discovers that of all his followers, only one boy still believes in him.

Om sets about returning his flock to the "way of Om", but as the boy believes he is going mad and seeing a talking tortoise, this isn't easy. The rest of his followers are going through a "Spanish inquisition" phase of their religion and Om needs a miracle to save the boy and restore his followers and hence; his power. He achieves this by dropping from an eagle and landing in the chief inquisitor's skull. This seems to satisfy the crowd and Om is restored to full strength, where-upon he returns to the home of the gods, Dunmanifestin and immediately starts a punch up with the other gods, who failed to come to his aid.

Om is mentioned in other Discworld books, either as a god, or appearing briefly in Dunmanifestin, but is never again a central character.