Oswiu in the context of List of monarchs of Northumbria

The Synod of Whitby was a Christian administrative gathering held in Northumbria in 664, wherein King Oswiu ruled that his kingdom would calculate Easter and observe the monastic tonsure according to the customs of Rome rather than the customs practised by Irish monks at Iona and its satellite institutions. The synod was summoned at Hilda's double monastery of Streonshalh (Streanæshalch), later called Whitby Abbey, in modern day Yorkshire.

Northumbria, a kingdom of Angles, in what is now northern England and south-east Scotland, was initially divided into two kingdoms: Bernicia and Deira. The two were first united by King Æthelfrith around the year 604, and except for occasional periods of division over the subsequent century, they remained so. The exceptions are during the brief period from 633 to 634, when Northumbria was plunged into chaos by the death of King Edwin in battle and the ruinous invasion of Cadwallon ap Cadfan, king of Gwynedd. The unity of the Northumbrian kingdoms was restored after Cadwallon's death in battle in 634.

Another exception is a period from about the year 644 to 664, when kings ruled individually over Deira. In 651, King Oswiu had Oswine of Deira killed and replaced by Œthelwald, but Œthelwald did not prove to be a loyal sub-king, allying with the Mercian King Penda; according to Bede, Œthelwald acted as Penda's guide during the latter's invasion of Northumbria but withdrew his forces when the Mercians met the Northumbrians at the Battle of Winwaed. After the Mercian defeat at Winwaed, Œthelwald lost power and Oswiu's own son, Alchfrith, became king in his place. In 670, Ælfwine, the brother of the childless king Ecgfrith, was made king of Deira; by this point the title may have been used primarily to designate an heir. Ælfwine was killed in battle against Mercia in 679, and there was not another separate king of Deira until the time of Norse rule.

Oswald (Old English pronunciation: [ˈoːzwɑɫd]; c 604 – 5 August 641/642) was King of Northumbria from 634 until his death, and is venerated as a saint, of whom there was a particular cult in the Middle Ages.

Oswald was the son of Æthelfrith of Bernicia and Acha of Deira and came to rule after spending a period in exile. After defeating the Welsh Gwyneddian ruler, Cadwallon ap Cadfan, at the Battle of Heavenfield, Oswald brought the two Northumbrian kingdoms of Bernicia and Deira once again under a single ruler and promoted the spread of Christianity in Northumbria. He was given a strongly positive assessment by the historian Bede, writing a little less than a century after Oswald's death, who regarded Oswald as a saintly king; it is also Bede who is the main source for present-day historical knowledge of Oswald. After eight years of rule, in which he was the most powerful ruler in Britain, Oswald was killed in the Battle of Maserfield while fighting the forces of Penda of Mercia, who then himself was defeated by Oswald's brother Oswiu.