Om is an omnipotent, omnipresent (only within the boundaries of the Omnian church) god in the country of Omnia. His temple resides in Kom, presumably the capital, and his followers are known as Omnians.

Unlike the major Discworld gods, who exist as a pantheon, Om is a monotheistic deity whose followers insist that he is the one and only true God. Despite all evidence to the contrary, Omnians also insist that the world is round. Omnianism is the most oft-mentioned religion on the Discworld, and its members are known for their fervent evangelism.

Omnianism used to be an intolerant religion, not because Om was an intolerant god, but because he was largely an indifferent one. After spending some time trapped in the shape of a tortoise his perspective was changed, and he allowed Brutha to turn Omnianism into one of the Discworld's more tolerant religions, although they still insist Om is the only real god.

Visit, a constable in the Ahnk- Morpork city watch, has the full name of 'Visit-The-Infidel-With-Explanatory-Pamphlets'; many modern Omnians are given similar names, such as 'Smite-The-Unbeliever-With-Cunning-Arguments'. This in contrast to older Omnians, who were given bloodier names. The names parody Puritan names like "O Be Joyful", "Fear-the-Lord" and "Job-Raked-Out-of-the-Ashes". (The ancestor of Samuel Vimes, Suffer-Not-Injustice, is also named in the Puritan manner, based as he is on the very Puritanical Oliver Cromwell).

The Omnian Quisition (consisting of the Inquisition and the Exquisition) used the Torquus Simiae Maleficarum ("The Monkey Wrench of Witches"; a reference to the real world Malleus Maleficarum or "Hammer of the Witches") to identify witches. The Book of Om says that witches shall not be allowed to live, although this may be a typo since it also says that they may be caught in traps of treacle. This has led some to believe the word may in fact be cockroaches.