In 1914, following the outbreak of the First World War, he took a staff role as assistant quartermaster-general and then assistant adjutant-general with the British Expeditionary Force. Following heavy fighting, he was assigned to 6th Division on 1 October as its assistant adjutant-general and quartermaster-general, succeeding Walter Campbell. He remained with the division until February 1915, handing over his post to Reginald May. O'Gowan was then transferred to a field command, taking over the 13th Brigade in the 5th Division. The brigade was temporarily attached to 28th Division at this time, and involved in defending against a heavy German attack at St. Eloi on 14 March, and the successful attack on Hill 60 in mid-April. In August, he was promoted to temporary major general returned to England to take command of the newly formed 31st Division, a New Army division predominantly drawn from the industrial towns of Northern England, and mainly composed of close-knit "Pals battalions".