Saint Oudoceus (known in modern Welsh as Euddogwy) is generally known as the third Bishop of Llandaff. In reality, he was probably a 7th century 'Bishop of Teilo' based at Llandeilo Fawr. Both are in South Wales.

Oudoceus' 12th century hagiographic 'life' in the Book of Llandaff tells how he was the son of King Budic of Brittany, born in that country shortly after his father's returned there from exile in Dyfed. His mother was said to be the sister of Saint Teilo and Budic promised that Oudoceus could train for a life in the Church under him. So Oudoceus came to Wales and eventually succeeded Teilo as Bishop of Llandaff.

Unfortunately, land grants, also in the Book of Llandaff, show that Oudoceus was a contemporary of a number of late 7th century kings in South Wales. He could not possibly have been the nephew or immediate successor of Teilo who flourished about a hundred years earlier. Furthermore, there is no evidence that Llandaff was the centre of a bishopric until at least the early 11th century. Oudoceus would have been a Bishop of Teilo, based at Llandeilo Fawr. Wendy Davies puts his episcopal reign between about 650 and 700. However, Oudoceus' associations with Llandaff are very strong and it seems likely that he was an early patron of the church there. He is said to have placed there a number of relics of Saint Teilo. He eventually retired to Llandogo, near Tintern, and there he died on 2 July. He was supposedly buried at the church in Llandaff, on the site of which the present Llandaff Cathedral now stands. He is one of the three saints to whom it is dedicated.