Oswiu, also known as Oswy or Oswig (c. 612 – 15 February 670), was King of Bernicia from 642 until his death. One of the sons of Æthelfrith of Bernicia, he became king following the death of his brother Oswald in 642. Unlike Oswald, Oswiu struggled to exert authority over Deira, the other Anglo-Saxon kingdom comprising medieval Northumbria, for much of his reign.
Oswiu and his brothers were raised in exile in the Scottish kingdom of Dál Riata after their father's death at the hands of Edwin of Deira, only returning after Edwin's death in 633. Oswiu rose to the kingship when his brother Oswald was killed in battle against Penda of Mercia. The early part of his reign was defined by struggles to assert control over Deira and his contentious relationship with Penda, his overlord. In 655, Oswiu's forces killed Penda in a decisive victory at the Battle of the Winwæd, establishing Oswiu as one of the most powerful rulers in Britain. He secured control of Deira, with his son Alhfrith serving as a sub-king, and for three years, Oswiu's power extended over Mercia, earning him recognition as bretwalda over much of Great Britain.
Oswiu was a devoted Christian, promoting the faith among his subjects and establishing a number of monasteries, including Gilling Abbey and Whitby Abbey. He was raised in the Celtic Christian tradition of much of the Irish world, rather than the Roman tradition practiced by the southern Anglo-Saxon kingdoms as well as some members of the Deiran nobility, including Oswiu's queen Eanflæd. In 664, Oswiu presided over the Synod of Whitby, where clerics debated over the two traditions, and helped resolve tension between the parties by decreeing that Northumbria would follow the Roman style. Oswiu died in 670 and was succeeded by his son, Ecgfrith.