Oesch was one of the Finnish Jaeger troops trained in the volunteer Royal Prussian 27th Jäger Battalion. During the Finnish Civil War he served as a battalion commander. Between the wars he trained in the French War Academy. He was Chief of General Staff between 1930 and 1940. He was promoted to Lieutenant General in 1936 and commanded the Aunus Group in the Continuation War.
He commanded the troops on the Karelian Isthmus in the decisive defencive battles of Tali and Ihantala in the summer of 1944.
Oesch received the Mannerheim Cross in 1944.