Olieslagers fascination with racing motorcycles led to a world championship in 1902. He was the first man to reach a speed of 100 km/h on a motorcycle. As his interest shifted to aircraft, he purchased a plane in 1909 and within four years, he had set seven world records. When the Germans invaded Belgium, he and his two brothers joined the army and donated their three Blériot XI monoplanes to the war effort.

In his first combat, Olieslagers attacked an enemy aircraft armed only with a pistol. Throughout the war, he rarely claim the enemy aircraft he destroyed. Despite his six confirmed victories, Olieslagers flew 491 sorties and had engaged in 97 dogfights. Returning to Antwerp at the end of the war, Olieslagers was responsible for the development of the Antwerp Airport in 1923.