The Oh God of Hangovers. His believers, if they can be called as such, are the people who wake up after a night's drinking and moan "oh god...", as they suffer from a hangover. Bilious continuously suffers from severe hangover, with headache, nausea, vision problems, fainting, and more. He looks like a young man with the classical toga, but his toga is colorful with bits of things that have come up out of him that he has not eaten or drunk the night before. Although hangovers are common, he is relatively minor among the Discworld gods. He hates people who drink a lot and never have a hangover because the hangover they're supposed to have goes to Bilious instead. He also hates people who drink. He hates most the god of wine, Bibulous, who is probably not even aware of Bilious's existence and is quite probably the main contributor to Bilious's hangovers since Bibulous always drinks and never has a hangover.
Technically, Bilious has existed since Bibulous was dreamt up. Admittedly, he was more of an omnipresent reservoir for hangovers before his manifestation, but something had to be getting the hangover.
Bilious was created from the excess belief that spilled over when Teatime killed the Hogfather, and may no longer exist, although Violet Bottler's belief in him may keep him going, as may the fact that gods can never truly die, even if they have no believers (Small Gods). However, if Violet's interest in Bilious was as shallow as the Senior Wrangler's interest in the Cheerful Fairy, who did disappear, Bilious may've suffered the same fate. It is also known that the Eater of Socks has survived in the Unseen University so there is a chance that Bilious is still around. It was during that time that he experienced, for the first time in his being (and, also, creating a paradox) the feeling of being sober, with a potion composed of many things, including Wow-Wow Sauce. Because he was sober, he was able to extract a measure of revenge against Bibulous - the constant hangover Bilious suffered from transfered over to Bibulous.
The oh god listened attentively to Susan Sto Helit, who was amazed. He didn't seem to disbelieve anything. She had never been able to talk like that before, and said so. "I think that's because I haven't got any preconceived ideas," he said. "It comes of not having been conceived, probably."
Appears in Hogfather.