Oswiu (c. 612 – 15 February 670), also known as Oswy or Oswig, is a King of Bernicia.
His father, Æthelfrith of Bernicia, is killed in battle, fighting against Rædwald, King of the East Angles and Edwin of Deira at the River Idle in 616.
Along with his brothers and their supporters, Oswiu is then exiled until Edwin's death in 633.
Following the death of his brother Oswald, defeated by Penda at the Battle of Maserfield on 5 August 642, Oswiu becomes King of the Bernicians.
He passes the next decade in obscurity as one of many kings subject to Penda.
In 655 Penda invades Bernicia, driving Oswiu before him.
The details of the campaign are unclear, but at the Battle of the Winwæd Oswiu unexpectedly defeats and kills Penda.
This victory is followed by Oswiu's short-lived imperium—he is traditionally counted as a Bretwalda—over much of Great Britain.
He establishes himself as King of Mercia, setting up his son-in-law, Penda's son Peada as a subject king.
Oswiu's unchallenged domination of Britain lasts only a short time, ending when a revolt among the Mercians establishes Penda's son Wulfhere as their king.
A negotiated settlement appears to have been preferred on both sides to prevent war.
Divisions within the Northumbrian church lead to the Synod of Whitby in 664, where Oswiu agrees to settle the Easter controversy by adopting the Roman dating.
His later years are marred by conflict with his son Ealhfrith.
Oswiu dies in 670 and is succeeded by his son Ecgfrith.
