King of Northumbria 642-670
born c 612 died 670
His early life
Oswiu's childhood was spent in exile with his elder brother Oswald in the northern kingdom of Dalriada. We know little of his whereabouts after that, but it must be presumed that he came south with Oswald in 642 when the latter defeated Cadwallon ap Cadfan at the battle of Heavenfield at acquired the throne of Northumbria.
Whilst Oswald was king, the only reference to Oswiu relates to his being married off to the Brythonic princess Rhianfellt of Rheged, which suggests he was playing some sort of role within Oswald's diplomatic manouevering. However in 642 Oswald was both defeated and killed by Penda of Mercia and his Welsh allies, as a result of which Oswiu succeeded his brother as king, presumably because Oswald's son Aethelwold was too young to rule.
The murder of Oswine
But Oswiu came to the throne of Northumbria in a position of weakness; his brothers alliance with the Gewissae had proved unfruitful and Mercia remained a powerful and threatening enemy, and whereas he was king in Bernicia he seems to have been unable to fully assert his authority in Deira where Oswine was established as king in 644.
Bede described Oswine as a "partner in the royal dignity" and "who governed the province of the Deiri" which implies a considerable degree of Deirian autonomy. Sufficient autonomy to be the cause of some degree of friction between the two kings. By 651 relations between Oswiu and Oswine had deteriorated to such an extent that both sides were preparing for war. At the last minute Oswine thought better of it, disbanded his army and went into hiding. Unfortunately for Oswine, he was betrayed captured,and put to death by Oswiu.
Although this counted as murder, Oswiu's only punishment seems to have been constructing a new monastery and installing a relative of Oswine's as the abbot. Whilst of course, the removal of Oswine allowed Oswiu to bring Deira under greater control where he appointed his nephew Aethelwold as king.
The problem of Mercia
Naturally enough Oswiu had to face the problem of Mercia; whose king Penda had after all been responsible for the death of three previous Northumbria kings, and would undoubtedly be aiming for the fourth.
Oswiu sought to placate and neutralise the Mercian threat by
negotiating a marriage between his son Alhfrith and the Mercian king Penda's daughter Cyneburh. As a result Alhfrith seems to have established a close friendship with Penda's son Paeda. A friendship that soon led to the both the marriage of Paeda to Oswiu's daughter Alhflaed and the baptism of Paeda at the hands of the Northumbrian bishop.
Oswiu also arranged with the king of Kent for a marriage with Edwin's daughter Eanfrith, (who was under Kent's protection). (1) This no doubt served to further bind together the dynasties of Deira and Bernicia as well as improving relationships with the kingdom of Kent.
War with Mercia
If pacifying Mercia had been Oswiu's policy it failed; Bede reports that Oswiu was subjected to savage and intolerable attacks
In 655 Penda brought an army into Northumbria, Oswiu tried to buy him off, but Penda rejected his approaches. And to make things worse, Penda was joined by Aethelwold, who rebelling against his uncle, no doubt had ambitions to follow his father as Northumbrian king. Oswiu had no option but to raise his own army. The two sides met at the battle of the river Winwaed, and although supposedly outnumbered three to one, Oswiu's forces won the day and Penda together with Aethelwold were both killed.
As a result of this victory Oswiu was at a stroke thrust into the position of pre-eminance; he was now effective ruler of both Northumbria and Mercia. He installed his son Alhfrith as the under king in Deira and his son-in-law, Paeda,(also the son of Penda) as an under king in Southern Mercia (2), whilst ruling northern Mercia himself.
The unravelling of his conquests
However Paeda was assainated within the year, and was forced to extend direct rule to the south. Three years later in 658, the Mercians rebelled, "expelling the officers of the foreign king, they at once recovered their liberty and their lands" and placed another of Penda's sons, one Wulfhere on the throne of Mercia.
Oswiu didn't quite have the military resources or perhaps the ability to hold on to his conquests; had he been able to do so, it might have been Northumbria rather than Wessex that came to dominate England. As it was, although Wulfhere was soon busy re-establishing Mercia as a power and viying once again for dominance with Northumbria Oswiu had done enough to persuade the Mercians that their ambitions were perhaps best pursued in the south.
The Synod of Whitby
There were a number of minor but significant differences between the practices of the Celtic tradition and that of the Roman tradition within the church (3) , the most controversial of which was a difference over the method for calculating the date of Easter. Northumbria followed the Celtic tradition (having been converted to Christianity from Ireland via Iona) but Oswiu's wife Eanfrith had been bought up in Kent within the Roman tradition.
This had the result that at times husband and wife celebrated Easter at different times. This was no doubt the source of a great deal of domestic inconvenience. Oswiu therefore convened a synod at Whitby in 664 to settle these issues. After some debate Oswiu himself issued the ruling that Northumbria should follow the Roman practice.
Although this decision effectively diminished the importance of Lindisfarne as a religious centre it did draw Northumbria back into the cultural mainstream and indirectly led to golden age of Northumbrian culture in the following century. A golden age of which the Venerable Bede was himself a product.
His achievments
Perhaps Oswiu's greatest achievment was that he died of natural causes in 670 aged about 58 years. That in itself, was a singular achievement in an age where the normal fate for any king was violent death at the hands of a rival.
He managed to hold the throne of Northumbria for twenty-eight years, generating a period of stability to the kingdom, (perhaps, responsible for finally uniting Deira and Bernicia at last), defended successfully against the threat of Mercia and brought the Northumbrian church within the mainstream European Roman Catholic tradition.
NOTES
(1) What exactly happened to his previous wife Rhianfellt or indeed the un-named Irish princess of the O'Neill who also bore him a son is not recorded.
(2) That part of Mercia to the south of the river Trent
(2) I use the phrases Celtic and Roman tradition rather than refer to a Celtic or Roman church in order to avoid giving the impression that somehow they were separate organisations