King of Northumbria (651–70). He succeeded his brother Oswald as ruler of the Northumbrian kingdom of Bernicia in 642 and incorporated the other northern kingdom, Deira, after arranging the assassination of his cousin Oswin (651). Although initially Northumbria was still in the control of Penda, king of the neighbouring Mercia, Oswy was able to defeat him (655) and establish himself as overlord of England. Oswy’s support of St Wilfrid at the Synod of Whitby (664), called to consider the rival claims of the Roman and Celtic forms of the Christian Church, was critical in gaining the decision in favour of Rome.