O'Grady was in Canada on the outbreak of war, eventually returning to England, where on 25 July 1916 he was commissioned as a second lieutenant (on probation) to serve in the
Royal Flying Corps. He did his basic flight training with No. 7 Reserve Squadron in September and October 1916, and was appointed a flying officer on 31 October. His training continued as part of "C" Squadron at the
Central Flying School, Upavon, Wiltshire, from October to December 1916, He was with No. 8 Reserve Squadron from December 1916 to February 1917, then returned to "C" Squadron at the Central Flying School, in March 1917. and appointed a
flight commander with the temporary rank of captain on 11 August. He gained six more victories by 26 August to bring his total to nine. His citation, published on 9 January 1918, read: :Second Lieutenant Conn Standish O'Grady, Royal Flying Corps, Special Reserve. ::"For conspicuous gallantly and devotion to duty in leading fighting patrols against superior numbers of enemy aircraft. He has himself brought down three enemy machines completely out of control, and others were seen to be destroyed, and his dash and determination when outnumbered by the enemy have continually won the highest praise." O'Grady was transferred to the RAF's unemployed list on 24 May 1919.
List of aerial victories ==Inter-war career==