When the Soviet invasion started the Winter War on 30 November 1939, Oesch continued as Chief of the General Staff at Finnish Supreme headquarters under Commander-in-Chief Mannerheim. Curiously, there's very little study of Oesch's role at the Supreme headquarters; he is usually left in Mannerheim's and Aksel Airo's shadow.
Oesch got the opportunity to show his talents as front commander in March 1940. The Red Army had surprised the Finns by crossing the frozen Bay of Viipuri and gained a foothold on its western shore. Mannerheim had created the Coast Group to repel the enemy, but its first commander Major General Kurt Martti Wallenius was dismissed in disgrace after holding the command for only three days. The situation was extremely critical, and Oesch was appointed to deal with it. Finnish defenses consisted mainly of badly equipped coast defense battalions manned by older reservists and battalions hastily transferred from Lapland. Oesch was able to hold this motley and weary force together until the end of the war on 13 March 1940, causing heavy losses for the Red Army and significantly slowing its advance. Mannerheim began to regard Oesch as a man who could deal with difficult situations.
During the ensuing peace, known as the Interim Peace by Finns, Oesch first returned to his previous post as the Chief of the General Staff for a few weeks, until taking the command of II Army Corps in April 1940
At the start of the Continuation War in June 1941, Oesch's army corps became the IV Army Corps. Its mission was to advance into the southern Karelian Isthmus. But Mannerheim gave priority to Lieutenant General Erik Heinrichs’s Army of Karelia, which advanced into northern Karelia north of Lake Ladoga. Oesch didn’t get permission to go on the offensive until 20 August 1941, almost two months after the start of the war. Oesch and his chief of staff Colonel Valo Nihtilä decided to start the attack two days later.
Once the IV Army Corps's offensive started, it made rapid progress. But Oesch became overstressed with work, and was forced to take a fortnight’s sick leave on 25 August 1941. However, on Nihtilä’s request, Oesch returned already on 30 August 1941, because Oesch’s deputy, Major General Taavetti Laatikainen, had neglected his new duties. On the previous day the IV Army CPost too long. Click here to view the full text.