The Venerable Bede (III, 6) says that Oswald had a greater dominion than any of his ancestors, and that "he brought under his sway all the nations and provinces of Britain, which are divided into four languages, namely the Britons, the Picts, the Scots, and the English". He had great power in the North-West, as far south as Chester and Lancashire, and was probably owned as overlord by the Welsh Kingdom of Strath Clyde, as well as by the Picts and Scots of Dalriada. In the East he was supreme in Lindsey, and the words of Bede seem to imply that he was overlord of Mercia, which was still ruled by Penda; but this could have been scarcely more than nominal.
At the battle of Maserfeld, said to be seven miles from Shrewsbury, "on the border of Wales, near Offa's dyke", Oswald was slain on 5 Aug., 642, and thus perished "the most powerful and most Christian King" in the eighth year of his reign and in the flower of his age. His last words were for the spiritual welfare of his soldiers, whence the proverb: "
God have mercy on their souls, as said Oswald when he fell." His body was mutilated by Penda, and his limbs set up on stakes, where they remained a full year, until they were taken away by Oswy and given to the monks at Bardney in Lindsey. In the tenth century some of the bones were carried off by Ethelred and Ethelfleda of Mercia to St. Peter's, Gloucester. His head was taken from the battlefield to the church of St. Peter in the royal fortress at Bamborough, and was afterwards translated to Lindisfarne, where, for fear of the Danes, it was placed in 875 in the coffin of St. Cuthbert, which found its resting place at Durham in 998. It was in the coffin at the translation of St. Cuthbert in 1104, and was thought to be there when the tomb was opened in 1828. His arm and hand (or hands) were taken to Bamborough and perhaps afterwards removed to Peterborough, and were still incorrupt in the time of Symeon of Durham, early in the twelfth century. Reginald gives an account of his personal appearance: arms of great length and power, eyes bright blue, hair yellow, face long and beard thin, and his small lips wearing a kindly smile.