Olaus Petri first studied at the University of Uppsala, then in Leipzig, and then finished his education with his M.A. at the University of Wittenberg in 1518. During his time in Germany he got acquainted with the main characters of the German reformation, Philipp Melanchthon and Martin Luther.
He returned to Sweden with his brother in 1519. It is reported how they both nearly died as the boat ran ashore on the island Gotland during a storm. They stayed on Gotland for a while and the bishop there, Mattias Gregersson, appointed Olaus as his secretary and as dean in 1520.
Olaus accompanied the bishop to Stockholm in 1520, to the tumultuous crowning of the Danish King Christian II of Denmark who had taken over Stockholm and held it for a year until he returned to Denmark. At the notorious Stockholm Bloodbath in November several Church men and politicians were executed. Olaus expressed his outrage and was as a result almost executed himself, but a German who had seen him in Wittenberg saved him by explaining he was a German.
When the Swedish King Gustav Vasa was crowned in 1523, Olaus got to make his acquaintance. A year later he moved to Stockholm as town secretary. He spoke out sharply in favour of Lutheranism and against the prevailing Roman Catholicism.
In October 1524 he and his brother were excommunicated by the cathedral chapter in Uppsala, on the grounds of heresy. It appears as though this did not trouble him, partly because he still had a strong supporter in the Swedish King. In 1525 he married, and had the mass sung in Swedish for the first time; one of Luther’s ideas.
Almighty God, who by the hands of Olavus and Laurentius Petri didst give to the people of Sweden the Scriptures and the services of the Church in their own tongue: mercifully grant that all men everywhere may be enabled to hear and read the Good News of salvation preached to them in terms that they understand, that so they may be drawn to the kingdom of thy Blessed Son, Jesus Christ our Lord, who liveth and reigneth with thee and the Holy Spirit, one God, now and for ever.
In 1531 the King declared Sweden to be Lutheran. This was mainly a result of the Petri brothers’ efforts to teach the King and the population about Luther’s tenets.
In 1539 Petri was ordained priest. But shortly thereafter he and the king Gustav Vasa had a falling out. Officially, the charge was for treason: Olaus had gotten knowledge of a conspiracy against the King during confession but had not revealed it. Another suggested reason is that neither wanted to give in to their convictions—Gustav wanted to control the church, while Olaus wanted it to have a degree of self-government. During trial on January 2, 1540, a court sentenced him and Laurentius Andreae to death.
After much bargaining and many requests, Olaus’ friends managed to get him released on bail. He became inspector over the schools in Stockholm 1542 and dean of Storkyrkan in Stockholm in 1543, wherein he also was buried at his death. Since 1898, there is statue of him outside Storkyrkan.
He was described as energetic and pushy and serious about matters in which he believed. He also wrote much, in fact he is responsible for a main part of the early printed literature in Sweden. Among others he wrote a Chronicle of Sweden, which despite not being altogether correct contains many interesting facts. He was also responsible for the first Swedish translation of the New Testament (1526), and had an important part in the translation of the whole Bible (Gustav Vasa Bible, 1543).