Olaf married the daughter of Causantín mac Áeda. He also allied himself with Eógan I of Strathclyde. In 937, Olaf led his allies into battle against Athelstan, king of England, in the Battle of Brunanburh and was decisively defeated.
After Athelstan's death in 939, Olaf again invaded York the same year, forcing Athelstan's successor, Edmund, into a treaty which ceded to Olaf Northumbria and part of Mercia. He did not get to enjoy his new lands for long, dying just two years later in 941. He was succeeded by Olaf Cuaran.
Under Olaf, Jórvík over-reached its own capacity of self-government. Despite the most extensive size and power wielded during his reign, the territorial framework of his kingdom was in sharp decline.