One of the hugely-bearded Great Prophets of the Omnian church. He came back from the desert claiming that the Great God Om had dictated to him all 193 chapters of the Book of St Ossory, claiming that Om spoke to him as a Pillar of Flame. This was later disputed by Om himself, in conversation with Brutha. Others may have disputed it at the time - but who was going to argue with anyone coming in from the desert after a prolonged stay there during the mushroom season...and with their eyes spinning in different direction?
In his book are the Directions, the Precepts, the Gateways and the Abjurations. Many of these were rendered useless by Brutha when he became Cenobiarch. However, since the Omnian Church schisms so often, some people (notably Constable Visit in Hogfather) still regard this book as canon.
He travelled with the Holy St Bobby - a donkey which was elevated to the bishopric. And thus made a righteous ass.
Interestingly, Ossory is also a region of Ireland traditionally known for werewolves. According to some stories, the werewolves of Ossory were kings of the region. Two werewolves (a married couple) from Ossory appear in Gerald of Wales' History and Topography of Ireland (1185 C.E.).