Olof was king in Sweden when Catholic missionary St. Ansgar made his second voyage from Germany to the Swedish city of Birka in the year 854 A.D.
Ansgar's biographer Rimbert in his Vita Ansgari (Life of Ansgar) relates that the Curonians in Latvia had rebelled against the Swedes and refused to pay them tribute. When the Danes heard of this, they saw the opportunity to take over the Swedish dominions in Courland. They marshalled a great fleet and sailed to Courland in order to take over their goods and to make the Curonians pay tribute to the Danes instead. The Curonians gathered forces from all five of their towns and butchered half the Danish army, after which they plundered the Danish ships, gaining a great deal of gold, silver, and other valuables.
When the Swedes and King Olof heard of the Danish failure, they decided to win the reputation that the Swedes could do what the Danes could not and to make the Curonians pay tribute again. Consequently, the Swedes gathered an enormous army and attacked Curland. The first town they attacked was called Seeburg. It had 7,000 armed men, but the town was pillaged, ravaged, and burnt by the Swedes. The Swedes sent back their ships and started out on a five-day expedition into the hinterland. They reached the town of Apulia (modern Apuole, where arrows and other traces of this battle were discovered by Birger Nerman) which had as many as 15,000 armed men.
The Swedes stormed the town but it was ferociously defended, and after eight days of battle many men had fallen without result. By the ninth day, the Swedes were weary of the battle and discussed whether to pursue it. They decided to cast lots, but without results. Rimbert then relates that some of the Swedes had heard of the Christian faith and they decided to cast lots asking the Christian god about how to proceed. They interpreted the results as full support from the Christian god and decided to attack the town once again.
When the Curonians saw that the Swedes were about to resume their attack, they gave up and offered to the Swedes all the gold, silver, and weapons that they had taken from the Danes the previous year. They also offered to pay half a pound of silver for every man in the town and to resume paying tribute to the Swedes and to give hostages as a warrant for paying. They declared that they wished to be the subjects of the Swedish kings as in former times.
King Olof granted their request, and the Swedes returned home with treasures beyond count and thirty hostages as a security for the Curonians' future loyalty.