His main work was the map of Northern Europe Carta marina and the Historia de Gentibus septentrionalibus, which is a commentary to the map. This work was the first geographical representation of the countries of the North in the Renaissance.
Magnus came to pass in 1524 in the wake of the Reformation as a devout Catholic, and his brother John Magnus, the exiled Archbishop of Uppsala, into exile. When he died, Olaus was called by Pope Paul III. appointed Archbishop of Uppsala, without this, however, could enter his territory. In this capacity he took part in the Council of Trent. As a great zealot for the Catholic faith and opponents of the Lutheran doctrine he took part in the burning of books in Bologna and was instrumental in the creation of books Index by Giovanni Della Casa with. In various inputs he tried leading men of the church to move to open the Counter-Reformation in the Scandinavian countries. 1534 transferred Pope Paul III. him the church and the associated Hospice Santa Brigida in Rome, where he opened a design center of his activities and various writings, including a historical work of his brother published.
After his death in 1557 he was at the altar of St.. Veronika buried in the Roman Basilica. When his Catholic successors, his nephew Magnus Laurentii was determined.