Born in Weißenfels in the Electorate of Saxony in 1735, Otto joined the Saxon army in 1753 as a cavalryman. In the Seven Years' War he fought at the battles of Kolín and the Breslau in 1757. He was also present at several sieges and skirmishes. He joined an Austrian Freikorps raised by his brother Wilhelm and participated in several successful ambushes and raids in 1760-1762.