During the early months of the First World War, he commanded Grenadierregiment "König Wilhelm I." (2. Westpreussisches) Nr. 7 in the field as its colonel. Future fighter ace Manfred von Richthofen witnessed the 22 August 1914, attack on Virton, Belgium, and wrote of Prinz Oskar’s bravery and his inspirational leadership at the front of his regiment as they went into combat. For this action, Oskar earned the Iron Cross, Second Class. A month later, at Verdun, Oskar again led his men in a successful assault into heavy combat, and was awarded the Iron Cross, First Class. After this action, he also collapsed and had to be removed from the field. Awarded the wound badge for his injuries, he spent much of the fall of 1914 recovering from what was reported to be a heart condition. He eventually returned to duty and served on the Eastern Front, where he was again awarded the wound badge.