He was born around 604 as the son of Æthelfrith of Bernicia and came to rule after spending a period in exile – he had fled to Scotland when Edwin seized the kingdom in 616 and had become a Christian at Iona. When Edwin died iun 633 he and his family returned to Northumbria, where Oscric and Eanfrid, his brother, were soon killed by the British king, Cadwalla, who ruled Northumbria as a tyrant for a year before.
Oswald defeated and killed him (with a much smaller army) at Hevenfelt near Hexham.
Adomnán in his Life of Saint Columba offers a longer account, which Abbot Ségéne had heard from Oswald himself. Oswald, he says, had a vision of Columba the night before the battle, in which he was told “Be strong and act manfully. Behold, I will be with thee. This coming night go out from your camp into battle, for the Lord has granted me that at this time your foes shall be put to flight and Cadwallon your enemy shall be delivered into your hands and you shall return victorious after battle and reign happily.”
Oswald described his vision to his council and all agreed that they would be baptised and accept Christianity after the battle. Soon after the battle Oswald sent for a bishop from Iona to preach the Gospel in Northumbria. First, a severe bishop was sent, who met with no success among the people whom he called barbarous and obstinate. He was soon replaced by the kindly Aidan, whose sermons Oswald imself interpreted and to whom he gave the island of Lindisfarne for a monastery and his episcopal seat – close to the royal residence of Bamburgh.
Oswald united Bernicia and Deira under his rule. After eight years of rule, in which he was the most powerful ruler in Britain, Oswald was killed in the Battle of Maserfield, at a place generally identified with Oswestry (although other candidates for the location of the battle have been suggested) in 642, and his body was dismembered as a sacrifice by order of Penda; the head, arms and hands were hung up on stakes.
His head was buried at Lindisfarne. His feast day is August 5th.
From the Oxford Dictionary of Saints and Wikipedia