He commanded the 47th Regiment during the Battle of the Sakarya. On his way, Osman Ağa and his regiment fought with Greek bands in Erbaa, Çakallı and Bafra. Osman Ağa's regiment was ambushed by Pontic bands in Havza and Merzifon, but ultimately destroyed by the regiment. During the war, almost all of the 42nd Regiment, consisting of 2000 people, lost their lives, including the commander Hüseyin Avni Alparslan. About 80–90 soldiers survived from the 42nd Regiment, who merged with 58th Regiment. 47th Regiment was ordered to capture a hill the army couldn't, but they didn't even have bayonets since they wore their traditional clothes. 47th Regiment attacked with their knives and succeeded to throw back Greeks, but sixty percent of them died. After the victory, Osman Ağa was honored with the rank of lieutenant colonel and the Medal of Independence.