While in Wittenberg with his younger brother Lars, Olaus met with and was influenced by the main characters of the German reformation, Philipp Melanchthon and Martin Luther.

Meanwhile, at the notorious Stockholm Bloodbath in early November, King Christian violated his promises of a general amnesty for the Sture party, and during the post-coronation festivities arrested and executed 80–90 churchmen and secular Swedish nobles, including Bishop Mattias.

[2] King Christian tried to appoint his friend and bishop of Odense, Jens Andersen Beldenak, to the now-vacant Strängnäs bishopric, but both Danes soon returned to Denmark, and the scholar-priest Laurentius Andreae, who had been named archdeacon in 1520, ran the diocese.

Olaus (whose father died in 1521, after which he and his brother had joined the insurgents under Vasa's leadership) attended the coronation, swore fealty to his monarch, and soon became the kingdom's chancellor.

[3] The Petri brothers' efforts to teach their king and fellow Swedes about Luther's ideas bore fruit the following year, as the Diet of Västerås declared Sweden Lutheran.

In fact, some consider the brothers and their ally Laurentius Andreae (who had physically crowned Gustav Vasa king in 1523) responsible for most early Swedish printed literature.

[7] After Olaus Petri and Laurentius Andreae were found guilty on January 2, 1540, both received death sentences, and even his brother Lars signed the judgment.

[2] However, after much bargaining and many requests, friends managed to get both clergymen released on bail (Stockholm's merchants contributing to pay the stiff fines), although clearly their political careers had ended.

Olaus received a royal pardon in 1542, possibly as a reward for completing the massive biblical translation project, and in that year was appointed inspector over Stockholm's schools.

[8] Part of the problem in assessing his life, however, is that accounts of the trial which ended his legal career were also published long after his death by Vasa's chronicler, Erik Jöransson Tegel.