When the Germans invaded Belgium, he and his two brothers joined the army as volunteers and donated their three Blériot XI monoplanes to the war effort. Jan was promoted to Sergeant, then commissioned before the end of 1914.
January 1915 saw him flying a Nieuport 10 and his fighter-pilot skills became evident and on September 12th, 1915 he became the first Belgian pilot, as well as one of the first pilots overall, to claim an aerial victory, when he forced down an Aviatik C.I.
Olieslagers habitually took the fight to the Germans and was indifferent to the paperwork to staking claims and he usually did not bother with claims for wins behind the German lines. All this lead to his poor record of approvals that was sufficient to keep his scores low.
He seldom took leave and tended to busy himself around his home aerodrome and the airplanes assigned to him. He would spend hours with rookie pilots, helping to ease their entry into the deadly art of aerial warfare.
On November 4, 1917, he fainted while landing and crashed onto Les Moeres aerodrome. He was taken to hospital in a coma, but aroused a few days later.
On November 9th, 1918, engine problems brought Jan Olieslagers down in a field near Eeklo. It was his 518th and final combat sortie. He had fought in 97 dogfights over a four-year stretch.
He finished the war with a stunted tally of six confirmed victories but this total could have been much higher had he took claiming victories seriously.
His awards included; Order of Leuplod (Belgian), Croix de Guerre (France), Legion of Honor (France), Order of Saint Stanislas (Russian) and Belgian War Cross.