Saint Olaf was born in 995, the son of a Norwegian lord named Harald Grenske, the great grandson of Harald Fairhair, and Asta Gudbransdatter. Olaf grew up in the household of his stepfather, Sigurd Byr of Ringarike. From the age of 12, he went on expeditions to the Baltic coast, Denmark and the Netherlands. Between 1009 and 1013 he fought under Thorkell the Tall against the English at London, Ringmere and Canterbury. For a time he was a captain of mercenaries for Duke Richard of Normandy, and in 1013 or 1014 he was converted to the Faith of Christ and baptized in Rouen. Then he entered the service of the exiled English King Ethelred and followed him back to England, where he fought on the English side at the taking of London Bridge. When the Danish King Canute conquered England, Olaf joined his service.
Over time, Canute grew jealous of Olaf, so that in 1015 Olaf and bishop Sigfrid [later canonized as a saint and enlightener of Sweden] went to Norway, where Olaf succeeded in seizing the kingdom in spite of much opposition. First, by distributing money, and with the support of his kinsmen on the Opplands, he gained control of Ostland. Then, on Palm Sunday, March 25th, 1016, he conquered the country's principal chieftains, Sven Hakonsson Jarl, Einar Tambarskjelve, and Erling Skjalgsson, in the sea battle at Nesjar (between Larviksfjord and Lengesundsfjord). In the same year he was accepted as King at the Oreting in Trondelag.
He had a comparatively peaceful reign for almost 10 years, and during this period considerably advanced the unification of Norway. Olaf was the first high king who secured real control over the inland areas of Trondelag and Opplandene. Moreover, he gained a foothold for the Norwegian national kingdom on the Orkney islands and Hjaltland.
With the aid of his English missionaries he succeeded in making Norway Christian. In 1024, Norway acquired a nationwide ecclesiastical organization with churches and priests, a Christian legal system and a first organization of the Church's finances. The king established peace and security for his people, remaking old laws and insisting on their execution, unaffected by bribes or threats. He built many churches, including one dedicated to Saint Clement at the capital, Nidaros (Trondheim). All other faiths except Christianity were outlawed.
At the beginning of his reign Olaf did not enjoy good relations with Sweden because the Swedish King Olof Skotkonung had seized a portion of Norway in about the year 1000. However, through the mediation of Saint Anna, King Olof's daughter, it was agreed that Olaf should marry his other daughter Astrid, and relations between the two Christian kings were restored. In this way the foundations were laid for the Christianization of the whole of Scandinavia.
After the death of the King Olof in 1022, Olaf made an alliance with his son Anund Jacob against Canute of England and Denmark. But in 1026 the allies were defeated by Canute at Helgean in Skane, Sweden.
Canute was able to bribe Olaf's court, who did not like him for his simplicity and gentleness, justice and piety, and they turned on their king. So in 1028, Canute sailed to Norway and drove out King Olaf and subjected Norway to himself.
Olaf decided to flee to Sweden and then to the court of his kinsman, Yaroslav of Kiev, whose father, the famous Saint Vladimir.
Early in 1030 Olaf set off for Norway over the frozen Russian rivers, in hopes of conquering his kingdom back. He sailed to Gotland with 240 men. King Anund of Sweden gave him 480 more, but when he faced Canute's army at Stikrlarstadir, he had no more than 3600 men against a peasant army of 14400 men. But Olaf's army was defeated and he himself was killed.
But immediately a great fear fell on the soldiers of Canute's army. And then miracles began to be manifested at Saint Olaf's body, and the body was found to be incorrupt. Soon the penitent Norwegians expelled the Danes, and recalled Olaf's son Magnus from Russia to be their king.
King Canute made no opposition to the veneration of Saint Olaf, and churches dedicated to the saint were soon being built throughout the Viking world, from Dublin to the Orkneys to Novgorod. Forty ancient churches were dedicated to Saint Olaf in Britain, and his feast occurs on several English calendars.