He served with distinction in the rank of Second Lieutenant and then First Lieutenant in most of the battles of the 1st Brigade of the Polish Legions. Initially a company commander in the 1st Regiment, in 1915 he was promoted to the rank of Captain and became a battalion commander within the 5th Regiment. After the Oath Crisis of 1917, as an Austro-Hungarian citizen, Olszyna-Wilczyński was drafted into the Austro-Hungarian Army and dispatched to the Italian Front, along with many of his colleagues. He commanded infantry platoons within 50th, 62nd and 59th platoons, after which he was transferred to Ukraine as a commander of the 3rd battalion of the 16th Regiment. There, he organized a cell of the Polish Military Organization.