The Royal Inniskilling Fusiliers formed part of 29th Division, intended for service in the Gallipoli Campaign. Now a captain in the 1st Battalion, he commanded a company during the landing at X Beach on the Gallipoli peninsula on 25 April 1915 and acquitted himself well during the early stages of the fighting. On 18 June 1915, the Turks mounted an attack on positions adjacent to those of O'Sullivan's company, forcing the troops manning the defenses to abandon it. He led his company in a counterattack to reclaim the lost position which exchanged hands several times during the next few hours. The commanding officer in the area, Brigadier General W. R. Marshall, eventually directed O'Sullivan to lead a party of Inniskilling and South Wales Borderers soldiers to capture the position which was achieved at dawn the following day.Snelling, 1999, pp. 136–139
