He served as captain of militia in the Fall of Antwerp, wherein the city surrendered to the Duke of Parma. After this point he moved to Middelburg. In January 1588, Van Os married Margretha van de Piet. He and his brother Hendrick sailed to obtain leather, grain and precious stones, primarily in the Levant and cities on the Baltic Sea. In 1595, he left on an expedition with Isaac le Maire to get salt in Setúbal. Van Os acquired more than one-seventh of the land in Beemster.