Olaf first conclusively appears in contemporary records in 933 when the annals describe him plundering Armagh on 10 November. He is then recorded as allying with Matudán mac Áeda, overking of Ulaid and raiding as far as Sliabh Beagh, where they were met by an army led by Muirchertach mac Néill of Ailech, and lost 240 men in the ensuing battle along with much of their plunder.[nb 2] An earlier reference to a "son of Gofraid" who plundered the monastery at Kildare in 928 might refer to Olaf but no name is given. Olaf's father Gofraid ua Ímair, King of Dublin died in 934 and Olaf succeeded him as king. The following year Olaf carried out a raid at Lagore crannog in County Meath, and then looted the burial chamber at Knowth the following week.
Olaf is described as "Lord of the Foreigners" by the Annals of the Four Masters in 937, at which time he went to Lough Ree and captured Amlaíb Cenncairech, King of Limerick and his troops after breaking their boats. This conflict can be ascribed to rivalry between the competing Viking settlements of Dublin and Limerick, with this event marking victory for Dublin. This period is considered to be the high-point of Viking influence in Ireland. Having secured his position in Ireland, Olaf turned his attention to England and Northumbria, which had once been ruled by Olaf's father and had been conquered in 927 by Æthelstan of England. Olaf allied with Constantine II of Scotland, whose kingdom had been invaded by Æthelstan in 934, and in 937, the same year as the victory over Limerick, Olaf and the Vikings of Dublin left for England.
The allied forces of Olaf and Constantine met the forces of Æthelstan at the Battle of Brunanburh, at a site which is the subject of much debate, although current scholarly consensus identifies the site as Bromborough in Cheshire. Olaf and Constantine commanded the Viking troops while Æthelstan alongside his brother Edmund led the English troops into the battle. Contemporary accounts indicate both sides suffered many casualties but the result was a decisive English victory. Olaf and Constantine survived the battle and returned to Ireland and Scotland respectively, however one of Constantine's sons died. The battle is well-attested, with references in Irish chronicles, and a poetic telling of the battle in the Anglo-Saxon Chronicle. The numerous references to it in various chronicles throughout the British Isles testify to its perceived importance at the time.
The annals record Olaf's return to Ireland in 938 as well as a raid he carried out that year on Kilcullen in modern-day County Kildare, where he is said to have taken a thousand prisoners. Æthelstan died in October 939 and very soon afterwards Olaf left for York where he was able to quickly establish himself as king of Northumbria. Olaf was joined in England by his cousin Olaf Cuaran, and Olaf's brother Blácaire was left to rule in Dublin while he was away. Symeon of Durham's Historia Regum records that Olaf and the new English king Edmund met at Leicester in 939 and came to an agreement on dividing England between the two of them. This peace was short-lived and within a few years of the agreement the Vikings had seized the Five Boroughs of Derby, Leicester, Lincoln, Nottingham and Stamford.[nb 3]
In 941 the Chronicle of Melrose records that Olaf raided an ancient Anglian church at Tyninghame in what is now the Scottish Borders and at the time was a part of Northumbria. This attack may have been more than just a raid, and may have been intended to secure a route through Scotland upon which communication between York and Dublin was reliant. Olaf died in 941 and he was succeeded in Northumbria by Olaf Cuaran. In recording his death, the annals title him "king of Danes" (Chronicon Scotorum) and "king of the Fair Foreigners and the Dark Foreigners" (Annals of Clonmacnoise).