After Oswald's death, the Northumbrian kingdom was divided. Bernicia came under the rule of Oswald's brother Oswy and the Deira was inherited by Oswine who was the son of one of the two kings that had ruled for a year in Northumbria after the death of Edwin. For a time there was peace between the kingdoms as they were under thrall to Penda of Mercia who had defeated Oswald. However, in 651 AD Oswy moved against the southern kingdom, declaring war on it's king Oswin. Bede records the events that followed: "Oswin realised that his opponents forces were far stronger than his own, and decided not to risk an engagement but to await a more favourable opportunity. So he disbanded the army that he had raised…and sent all his men to their homes. He himself, accompanied by a single trusted soldier named Tondhere, went back and lay concealed in the house of the nobleman Hunwald, whom he regarded as his greatest friend. Alas, it was far otherwise: for Hunwald betrayed Oswin and his man to Oswy, who amid universal disgust ordered his commander Ethelwin to put them both to death."
Clearly, Oswy was no saint. He was also troubled by the persistence of Penda who brought a campaign against Northumbria in 655AD. Penda drove Oswy's forces ahead of him and took the fight at least as far as northern Bernicia , where Oswy was forced to offer great treasures and hostages. This appeased the Mercian king for a time, but their armies clashed again later that year at the Battle of the Winwaed. Penda was killed along with many of his British allies and Oswy seized his chance to claim d ominion over the defeated Mercians. His son in law Paeda was installed as client king and Oswy solidifed his dominance of a vast territory, earning his title of Bretwalda. His success was cut short when Paeda was killed by the sons of Penda and Oswy was forced to concede the Mercian kingdom, but as king of Northumbria he was still formidable. Oswy's most lasting achievement was to preside over and ultimately determine the outcome of the Synod of Whitby in 664 AD
It is no accident that the most crucial meeting of the new Christian church leaders was held at Whitby, in the province of Deira. The Northumbrian dominance of the early church is readily apparent. However, the Northumbrian traditions varied from those of the church in Rome and the Whitby gathering was intended to redress this imbalance and agree on crucial interpretations, most notably the timings of the important Christian festivals such as Easter. Oswy decided to adopt the Roman calendar, and to this day our modern western calendar retains this dating.
The 7th century was clearly a time where Christian Orthodoxy was still being debated. Even in the cemetery of Bamburgh, there are a bewildering array of burial types and varying positions. Burials provide some of the best archaeological evidence for the prevailing belief systems of a culture. How the dead were treated is a reflection of the individual and the cultural milieu as well as the personal circumstances of death, be it peaceful or violent, and whether it was a representation of orthodox practices or social taboos such as those with criminal or superstitious associations.
By Oswy's death in February 670 AD, the Christian dominance of the Northumbrian royal house was irrefutable. Oswy was buried at Whitby Abbey alongside Edwin, and his wife and later his daughter became Abbess there before their own death and burial at the site.
Oswy was succeeded by Ecgfrith, who was later succeeded by Aldfrith as Northumbrian overlord.