An Omnian is someone who practices a religion that appears on the Discworld, created by Terry Pratchett. They have got quite good at it by now.

Omnians are a growing religion on the Discworld. Founded by their god Om, and renewed by his prophet Brutha in Small Gods, they now are a major religion. Their belief in only one god as well as their methods, e.g. inquisition in Small Gods, make this religion appear as a mirror of early Christianity. A major factor in Omnian affairs at this time was that very few people actually believed in Om, although they all believed in what the Quisition did to unbelievers.

In the newer books, following Brutha's reformation, the church has become more evangelical in its methods, and you can see its followers going from door to door to convice unbelievers. Omnians of this period have names like "Smite-The-Unbeliever-With-Cunning-Arguments". Owing to Brutha's tolerance for opposing viewpoints, the church also schisms every couple of weeks.

The religion is based in the desert country of Omnia, a theocracy on the Klatchian continent. Omnia is ruled by the Cenobiarch. At the time of Small Gods the Cenobiarch was a very old man, and the country was actually ruled by his advisors, chiefly Vorbis .