Oswald (c. 604–August 5, 641/42) was the King of Northumbria from 633 or 634 until his death. The son of Æthelfrith of Bernicia, King of Northumbria, he is best remembered as a Christian martyr.

After his father was defeated and killed by Raedwald of East Anglia, Oswald fled to Dalriada, where he was converted to Christianity by the monks of Iona. He fought under Connadd Cerr in the Battle of Fid Eoin in Ireland.

After the king of Gwynedd, Cadwallon ap Cadfan (in alliance with Penda of Mercia), killed King Edwin in battle at Hatfield Chase in 632 (or 633), Northumbria was split between its sub-kingdoms of Bernicia and Deira. Oswald's half-brother Eanfrith became king of Bernicia, but he was killed by Cadwallon in 633 (or 634) after attempting to negotiate peace. Oswald then returned from exile with an army and marched against Cadwallon; his ranks were bolstered by Scots sent by the king of Dalriada, Domnal Brecc. The day before the two sides met in battle at Heavenfield, Oswald made his soldiers construct a wooden cross. He knelt down, holding the cross in position until enough earth had been thrown in the hole to make it stand firm. He then prayed and asked his army to join in. In the battle that followed, the Welsh were routed despite their superior numbers and pursued for miles by the Northumbrians; Cadwallon himself was killed.

Following this victory, Oswald reunited Northumbria. He is considered to have been Bretwalda for the eight years of his rule (both Bede and the Anglo-Saxon Chronicle say that Oswald's reign was actually considered to be nine years, the ninth year being accounted for by assigning to Oswald the year preceding his rule, "on account of the heathenism practised by those who had ruled that one year between him and Edwin"), although his authority over the other Anglo-Saxon kingdoms seems to have been limited. He did, however, form an alliance with Wessex under Cynegils: Cynegils converted to Christianity and accepted baptism, and Oswald married Cyneburh, the daughter of Cynegils. With her he had a son, Æthelwald.

Although Edwin had previously converted to Christianity in 627, it was Oswald who did the most to spread the religion in Northumbria. It was he who gave the island of Lindisfarne to the bishop Aidan, who established a monastery there.

Oswald won some successes against the British to the north, but the primary concern of his reign was Northumbria's conflict with the rising power of Mercia under Penda. He was killed by the Mercians at the Battle of Maserfield in 641 or 642, and his body was dismembered by the pagan Penda. (Bede says that Oswald died in the thirty-eighth year of his age.) Oswald's head and limbs were placed on stakes, but according to legend, one of his arms was taken by his pet raven and dropped on a tree. The people quickly came to regard Oswald as a martyr and saint: a holy well of healing was said to have sprung up at the spot where the arm had landed, and the site soon became known as Oswestry, or "Oswald's Tree". His feast day is August 5.