During this period he copied paintings from the works by Paulus Potter and Charles Dujardin. Van Os was particularly fond of animals as his subject matter and made such an excellent copy of one of Potter"s works - Young Bulletin - that it was purchased by William V, Prince of Orange. After completing his training, he departed for Amsterdam, where he supported himself primarily by painting rather mediocre portrait miniatures and giving drawing lessons.
Around 1805 he began to devote himself to producing landscape paintings filled with his favourite subject of cattle and was still strongly influenced by the 17th-century Dutch masters.
In 1813 and 1814 he trained as a captain of volunteers and experienced military engagements. This led him to try military subjects in his art
Emperor Alexander I of Russia purchased a picture in this theme by Van Os and placed it in his palace at Saint St. Petersburg. Van Os died in the Hague in 1839.