His attempt to conquer Denmark brought the wrath of Cnut, who drove him away easily. The Battle of Helgeå in 1026 was lost decisively against the combined Danish and English force of Cnut, and Olaf fled to the Kievan Rus. When Cnut's lieutenant in Denmark died in a shipwreck a short time after, Olaf returned to Norway to re-take it from Cnut. His former subjects had had enough, however, and opposed him. This led to the Battle of Stiklestad, a farm in a valley north of Trondheim, in 1030.
According to the Anglo-Saxon Chronicle's solo entry for 1030:
A.D. 1030. This year returned King Olave into Norway; but the people gathered together against him, and fought against him; and he was there slain, in Norway, by his own people, and was afterwards canonized. [my emphasis]
Snorri Sturluson's Heimskringla offers more detail. He says Olaf received three major wounds, first in the knee and the neck, and then, while leaning against a large stone, he was killed with a spear thrust into his stomach by Thorir Hund, one of the Norwegian leaders opposing him. Olaf's body was buried near a river.
Some sources credit Olaf with the Christianization of Norway, despite the fact that most of what we can confirm involves fighting with other countries (and his own). A year after Stiklestad, however, he was disinterred and the coffin opened up, only to find that his body was uncorrupted—a sign of great holiness. The coffin was taken to St. Clement's Church in Trondheim.
Grimketel, an English bishop and missionary in Norway, began the process of beatification almost immediately. He likely wanted Norway to have its own saint ASAP. A century later, a cathedral was built on the site where Olaf's body was originally buried, and Olaf's body was transferred there and placed in a silver reliquary. (It's not there now: in the 16th century he was re-buried somewhere in the cathedral and the silver was melted down for coins.)
After Stiklestad, Cnut remained king for five years, leaving his first wife Ælfgifu in charge with their son, Svein. In 1035, Olaf's illegitimate son, Magnus "the Good" laid claim to the throne, and Ælfgifu and Svein fled to England. Tomorrow we'll see how things fared under Magnus.
