He joined an Austrian Freikorps raised by his brother Wilhelm and participated in several successful ambushes and raids in 1760-1762.
At the end of the war he formally entered the Austrian army, joining the Hesse-Darmstadt Dragoon Regiment # 19 as an Oberleutnant.
In 1793 during the War of the First Coalition, Otto joined the Austrian army in Flanders, serving at the siege of Valenciennes.
At the battle of Caesar's Camp on August 7, he commanded an infantry-cavalry brigade in the Count of Clerfayt's column.
In the ensuing Battle of Villers-en-Cauchies, Otto's troopers smashed a 7,000-man French division, inflicting 1,200 casualties.