Along with his brothers and their supporters, Oswiu was then exiled until Edwin's death in 633.

Oswiu then declared his judgment in favor of the holder of the keys, i. e. the Roman ( and Petrine ) practice.

Oswiu then ruled all Mercia himself.

Chad was invited then to become bishop of the Northumbrians by King Oswiu.

There was a royal marriage between Prince ( later King ) Oswiu of Northumbria and the Rhegedian princess Riemmelth, granddaughter of Rum ( Rhun ), probably in 638, so it is probable that it was a peaceful takeover, both kingdoms being inherited by the same man.

Indeed, this is noted as occurring in the household of King Oswiu of Northumbria, whose kingdom had been evangelized by both Irish and Roman missionaries.

Abels identifies several conflicts contributing to both the calling of the council and its outcome, including a generational conflict between Oswiu and Alhfrith and the death of the Archbishop of Canterbury, Deusdedit.

The Vita Sancti Wilfrithi states that, nominated by both Oswiu and Alhfrith, he was made bishop at York, and that he was a metropolitan bishop, but York at that time was not a metropolitan diocese.

However, there were diplomatic marriages between the two kingdoms: Æthelred's sister Cyneburh married Alhfrith, a son of Oswiu of Northumbria, and both Æthelred and his brother Peada married daughters of Oswiu.

Oswald's successor, Oswiu, married Eanflæd, who was Edwin's daughter and Eadbald's niece, thereby gaining both Deiran and Kentish connections.

Following the battle, Deira, in the southern part of Northumbria, chose a king of its own, Oswine, while Bernicia in the north ( which had been dominant, with Oswald, a member of the Bernician royal line, ruling both Bernicia and Deira prior to Maserfield ) was ruled by Oswald's brother Oswiu.

It is quite clear that Oswiu intended Chad to be bishop over the entire Northumbrian people, overriding the claims of both Wilfrid and Eata.

Edwin banished Æthelfrith's sons, including both Oswald and Oswiu of Northumbria.

The eighth-century monk and chronicler Bede lists both Oswald and Oswiu as having held imperium, or overlordship, over the other Anglo-Saxon kingdoms ; in Oswiu's case his dominance extended beyond the Anglo-Saxons to the Picts, the Gaels of Dál Riata, and the many obscure and nameless native British kingdoms in what are now North West England and southern Scotland.

Oswiu vowed to give his daughter Ælfflæd to the church, and to found a dozen monasteries if he was granted the victory, and assisted by Ealhfrith he engaged Penda with a small army in the Battle of the Winwæd, which took place in the region of Loidis, which is to say Leeds.

Although Oswiu himself had been brought up in the " Celtic " tradition, political pressures may have influenced his decision to call a council, as well as fears that if dissent over the date of Easter continued in the Northumbrian church it could lead to internal strife.

According to Bede, before the battle Oswiu prayed to God and promised to make his daughter a nun and grant twelve estates for the construction of monasteries if he was victorious.

If they did divide the kingdom, it is likely that Eowa ruled northern Mercia, as Penda's son Peada was established later as the king of southern Mercia by the Northumbrian Oswiu, who defeated the Mercians and killed Penda in 656.

Oswine and Oswiu came into conflict circa 651, Bede blames Oswiu for the troubles and writes: For when they had raised armies against one another, Oswin perceived that he could not maintain a war against one who had more auxiliaries than himself, and he thought it better at that time to lay aside all thoughts of engaging, and to preserve himself for better times.

However, Penda and his alliance was still a major threat, and they besieged Oswald's Bernician successor Oswiu at his fortress of Bamburgh in 650 or 651, though they did not defeat him.

Oswald and Oswiu returned to Northumbria after Edwin's death in 633, and between them they ruled for much of the middle of the seventh century.

Divisions within the Northumbrian church led to the Synod of Whitby in 664, where Oswiu agreed to settle the Easter controversy by adopting the Roman dating.

As well as learning the Scottish language and being thoroughly Christianised, Oswiu may have fought for his Gaelic hosts, perhaps receiving his arms — a significant event — from a King of Dál Riata, such as Eochaid Buide, son of that Áedán mac Gabráin whom his father had defeated at the Battle of Degsastan.

Welsh sources suggest that Oswiu campaigned in Wales in the late 650s, imposing tribute on the Welsh kings who had previously been Penda's allies such as Cadafael, the battle-dodging King of Gwynedd.

Bede writes that Oswiu had intended to undertake a pilgrimage to Rome in the company of Bishop Wilfrid.

To replace the departing ecclesiastics, Oswiu chose mostly Irishmen who were from the parts of Ireland that kept the Roman Easter ( as most of Ireland had done for some time by the 660s ).

Before Wilfrid's return Oswiu had appointed Ceadda in his place, resulting in Wilfrid's retirement to Ripon for a few years following his arrival back in Northumbria.

The church in Northumbria had traditionally used the former calculation, and that was the date observed by King Oswiu.

Because Oswiu knew that Alhfrith had been a supporter of Wilfrid's, Oswiu prevented Wilfrid's return, suspecting Wilfrid of supporting his rivals.

That Ceadda was supported by Oswiu, and Wilfrid had been a supporter of Oswiu's son, lends further credence to the theory that Alhfrith's rebellion took place while Wilfrid was in Gaul.

By a complex chain of reasoning one can deduce that Eanhere married Osthryth, daughter of Oswiu of Northumbria, and had sons by her named Osric, Oswald and Oshere.

The kings of Bernicia were thereafter supreme in that kingdom, although Deira had its own sub-kings at times during the reigns of Oswiu and his son Ecgfrith.

Oswiu, who was Oswald's brother but had succeeded him only in Bernicia, the northern part of Northumbria, was besieged by Penda's forces at a place called Iudeu ( identified with Stirling ) in the north of his kingdom.

Mercia's position of dominance, established after the battle of Maserfield, was destroyed, and Northumbrian dominance was restored ; Mercia itself was divided, with the northern part being taken by Oswiu outright and the southern part going to Penda's Christian son Peada, who had married into the Bernician royal line ( although Peada survived only until his murder in 656 ).

Caelin, the brother of Cedd and Chad, was chaplain to Ethelwald, a nephew of Oswiu, who had been appointed to administer the coastal area of Deira.

York later became the diocesan city partly because it had already been designated as such in the earlier Roman-sponsored mission of Paulinus to Deira, so it is not clear whether Bede is simply echoing the practice of his own day, or whether Oswiu and Chad were considering a territorial basis and a see for his episcopate.

Nevertheless, Bede cannot conceal that Oswiu and Chad had broken significantly with Roman practice in many ways and that the Church in Northumbria had been divided by the ordination of rival bishops.

Irish annals record the siege of Edinburgh, thought to have been the royal stronghold of the Gododdin, in 638, and this seems to mark the end of the kingdom ; that this siege was undertaken by Oswald is suggested by the apparent control of the area by his brother Oswiu in the 650s.

Oswald, who is known to have been Acha's son, was accepted as king in Deira, while Oswiu appears never to have ruled the kingdom directly.

Oswiu is known to have been married three times.

Nonetheless, following the Protestant Reformation, the events of the synod have been symbolically interpreted as a “ Celtic Church ” opposing a “ Roman Church ”, and the decision of Oswiu was thus interpreted as the “ subjugation ” of the “ British Church ” to Rome.

The reason for Wilfrid's delay has never been clear, although the historians Eric John and Richard Abels theorise that it was caused by Alhfrith's unsuccessful revolt against Oswiu.

In 655 Penda besieged Oswiu of Northumbria at Iudeu, the location of which is unknown but which may have been Stirling, in Scotland.

It has been suggested that the Mercian revolt succeeded because Oswiu may have been occupied with fighting in Pictland, in northern Britain.

The Tribal Hidage is difficult to date precisely ; it may have been written down in Wulfhere's reign, but other suggested origins include the reign of Offa of Mercia, or Edwin or Oswiu of Northumbria.

It has been suggested that he adopted Christianity as part of a settlement with Oswiu.

Breeze ( 2004 ) argues that Penda and his army would have been in a difficult strategic location along the Went during their withdrawal, giving Oswiu a good opportunity to attack.

However, while Oswiu had been beaten he had not been defeated.

With many of its leaders having been killed in battle, the alliance was caught unawares in a sortie by Oswiu at the Winwaed (, its location uncertain ).