He was the son of King Oswald of Northumbria, who was killed by Penda of Mercia at the Battle of Maserfield in 642. At Oswald's death Northumbria was reduced back to the two kingdoms of Bernicia and Deira. Oswiu was the king of Bernicia and was acknowledge as overlord of Deira.
Penda had placed Oswine of Deira[a] on the throne of Deira. But Oswine did not get along with his overlord Oswiu. He raised an army against Oswiu but then realized his army wasn't large enough and he sent them home. He was betrayed by one of his nobleman to Oswiu and was murdered in 651.