Oswy or Oswiu (both: ŏz´wē), d. 670, king of Northumbria. He succeeded (641) his brother Oswald in Bernicia only, Deira (the other part of Northumbria) having become a dependency of Mercia. However, when he killed Penda of Mercia at the battle of Winwæd (654), he not only made himself ruler of all Northumbria but gained actual possession of Mercia and overlordship of the southern English kingdoms. He lost Mercia again to Penda's son Wulfhere in 657. Oswy continued the conversion of England to Celtic Christianity (see Celtic Church), sending missionaries to Mercia and Essex. To resolve the differences between the Celtic and Roman usages of the church in England, the king called the Synod of Whitby (663), at which he gave his decision in favor of the Roman form of Christianity.