In the autumn of 1916, the 161st Battalion left Huron County and travelled to Halifax in preparation for going overseas. They embarked from Canada on November 1st and arrived in Liverpool on November 11, 1916. On December 1st, Private O'Brian transferred to the 64th Battalion and then in the middle of January 1917 he transferred to the 2nd Canadian Labour Battalion. He was with this unit when he went overseas in France on February 10, 1917. Six months later in August he transfers to the 58th Battalion and joins his unit in the field in very early November 1917.
The diaries of the 58th Battalion state one man was killed on April 23rd and another man was killed on April 24th. However, the Brigade diaries state the 58th Battalion did not lose a man on April 23rd but lost two men on April 24th and on this day reinforcements were required.
Between 6 am of April 23rd and 6 am of April 24th the artillery and trench mortars of the Brigade were very active against the enemy trenches and their wire system. The machine guns on both sides were also very active. On April 23rd the weather was good with light winds of 7 mph from the south. Earlier the 58th had relieved the 116th Battalion in the St. Emile Sector of the front. All four companies moved from the Lieven area to the front with "B" Company being left front and "C" Company on the right front. "D" Company was support and "A" Company was in reserve. During the morning the heavy guns and the trench mortars were very active against enemy positions. At approximately 6 pm enemy rifle and grenade fire in Nun's Alley forced a three man post to retire. The 58th then sent out a patrol from the Combination Sap and proceeded along the sap until they encountered the first belt of enemy wire. At this point a large enemy gun in the enemy support trenches became active and brought down fire upon the patrol but they could not locate the enemy gun. No enemy was encountered during the patrol which returned to their lines at 2:30 am on April 24th following a two hour patrol.
Private O'Brian was performing his military duties with the 58th Battalion during an attack in the trenches near St. Emile located north of Lens when he was killed in action.