After Poland regained its independence, on November 4, 1918, Olszyna-Wilczyński and his battalion (composed of Poles mostly) joined the Polish Army. An experienced officer, he was attached to the ad hoc formation of Col. Czesław Rybiński fighting in Volhynia during the Polish-Ukrainian War. On November 27 his company was defeated in a skirmish near Mikulińce near Tarnopol and Olszyna-Wilczyński himself was heavily wounded and taken prisoner of war by the Ukrainians. It was not until their defeat in June 1919 that he was released and allowed to return to the Polish Army. Promoted to Lieutenant Colonel, he became the commanding officer of the 3rd Polish Legions Infantry Brigade, and then (since September 28, 1919) the 1st Polish Legions Infantry Brigade. With the latter unit he took part in the Polish-Soviet War.