**Oswiu** (c. 611–15 February 670), also known as **Oswy** or **Old English: *Ōswīg***, became the King of Bernicia at the death of his brother Oswald. Later he became the King of Northumbria. When Oswiu defeated and killed Penda of Mercia at the Battle of the Winwaed, he became the seventh bretwalda over the southern English kings. He took the throne of Mercia and appointed his son-in-law Peada (Penda's son) as a client king. Oswiu was one of those rare kings of Northumbria who died of natural causes and not in battle.

Early career
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Oswiu was born c. 611 He was a son of King Athelfrith of Bernicia and his second wife Acha of Deira. He was a younger brother of King Oswald of Northumbria. When he was about 6 years old his father was killed in battle by Edwin who became King of Northumbria. Along with his brothers Eanfrith and Oswald and their sister Aebbe, he found safety with the Scots in Dál Riata. The monks of Iona baptized the children and raised them in the Celtic Christian faith.

About 634 Oswiu married Rhiainmelt, daughter of Rhoeth of Rheged, a marriage alliance arranged by his brother King Oswald. He had a son by this marriage, Alfrith of Deira. Rhiainmelt seems to have died as a year later Oswiu was in Ireland with the Dál Riatans. There he was involved with Fina, the daughter of the high King Colman. There is no record they married but they had a son Aldfrith of Northumbria (Irish name *Flann Fína mac Oswiu*).

King of Bernicia
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When his brother Oswald was killed in 642, the overlordship died with him. Oswald had been king of a combined Bernicia, Deira and Lindsey (kingdom). But Northumbria was divided back into its earlier weaker sub-kingdoms. Penda, now the most powerful king in England took Lindsey. Oswiu became King of Bernicia. Oswine claimed Deira, preventing Oswiu from keeping it with Bernicia. A year after it happened Oswiu recovered parts of his brother's body from the battlefield where he died. Oswiu had Oswald's arm enshrined in his royal centre at Bamburgh and his head was taken to Lindisfarne.

Oswiu invaded Deira in 651 seeking to reunite Deira with Bernicia. Oswine was betrayed by one of his own noblemen and was murdered in 651 on Oswiu's orders. But the Deirans chose Athelwald, Oswiu's nephew, as their king. Athelwald then aligned himself with Oswiu's enemy, Penda. Deira was under the protection of Mercia for the next three years. But Penda was determined to defeat Oswiu and take Bernicia as well. In 654 Penda raised a large army of some thirty 'legions'. According to Bede, Penda attacked Oswiu several times. With a much smaller force Oswiu attacked Penda's army near the banks of the River Winwaed. Most of the commanders of the thirty legions were killed including King Anna of East Anglia and Penda. From this time Oswiu became the bretwalda or overlord over all the southern English people including Mercia. Mercia was then divided. North of the River Trent was controlled directly by Oswiu. He made Peada of Mercia, Penda's son, king over the part of Mercia south of the Trent. Peada had married Oswiu's daughter, Alflaed. Peada was murdered five months later by Alflaed, possibly on Oswiu's orders. Mercians rebelled against Oswiu and Peada's brother Wulfhere became king of Mercia.

Easter controversy
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When Oswiu and Eanflaed were married, he was raised in the Celtic church while she was raised in the Roman church. The main difference was how each calculated the date of Easter. Members of the Celtic church might be celebrating Easter while the Roman church was celebrating Lent. In 658 his son Alfrith, who now supported the Roman Church, replaced several of his Celtic church officials with those from the Roman church.

To resolve the differences between the two religions in his kingdom, Oswiu called for a synod in 664. Called the Synod of Whitby it was to decide on when to celebrate Easter and what rules the monasteries in Northumbria would follow. Members of the church were summoned from all over the north. The Roman position was given by Wilfrid who would afterwards become Bishop of Northumbria. Bishop Colmán argued for the Ionan calculation of Easter given them by Columba, founder of their order. Oswiu made the decision in favor of the Roman practice, uniting Northumbria under one religion. It joined Northumbrian religious practices to those of the southern English.

Bede, in his *Ecclesiastical History*, Book IV, chapter 5 gives the date of Oswiu's death as 15 February, 670 and his age at 58. His wife Eanflaed retired to the monastery at Whitby. This is where their daughter, also named Enflaed, was brought up. In 680 Enflaed and her daughter became joint abbesses at Whitby. Enflaed died c. 704.

Family
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Based on Nennius' *Historia Brittonum* Oswiu's first wife was a Briton named Reinmelth. She was the daughter of Royth, King of Rheged. Together they had a son:

* Alfrith, King of Deira.
* Alflaed, she married Peada of Mercia.

  

By Fina, daughter of the Irish High King Coleman, he had:

* Aldfrith (*Flann Fína mac Oswiu*), King of Northumbria.

  

Oswiu married Eanflaed, daughter of King Edwin of Deira. Together they had:

* Ecgfrith, King of Deira under his father's rule and King of Northumbria at Oswiu's death.
* Elfwine, Kings of Deira.
* Ostryth, married Aethelred I of Mercia
* Enflaed, co-abbess of Whitby

Other websites
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* Kings of Northumbria[*permanent dead link*]
* The Battle of Winwaed: The Venerable Bede's Account