Extract from 'Australian Dictionary of Biography: 'On 30 October 1913 he joined the Postmaster-General's Department in Australia as a lineman and enlisted in the Australian Imperial Force as a sapper in the 4th Signal Troop, 4th Light Horse Brigade, in February 1915.
In Egypt his troop was allotted to the newly formed 2nd Divisional Signal Company and in August O'Brien accompanied it to Gallipoli. On the formation of the 4th Divisional Signal Company in Egypt in March 1916, O'Brien was transferred to it as a corporal and in June embarked for France where the company took over communications for the 4th Division at Merris on 11th June.
During operations at Pozieres he was awarded the Military Medal for laying and maintaining telephone lines under shell-fire, and for a similar act at Flers during the winter of 1916 he was awarded a Bar to the Military Medal. He was promoted sergeant on 10 April 1917 and for bravery while in charge of construction and maintenance parties on 7-14 June at Messines, Belgium, he was mentioned in dispatches.
During operations of the 4th Division at Ypres he showed remarkable disregard for danger and took charge of working parties constructing buried cable routes from Divisional Headquarters to the line brigades and supporting artillery. The scale of these cable-laying operations was enormous. The cable had to be dug to a depth of six feet (183 cm) to minimize the effects of shell-fire; it had to be dug at night and usually by working parties of tired infantrymen. For his efforts in these conditions his company commander, Major J E Fraser, described O'Brien as 'leaving no stone unturned to immortalize the password of the Signal Service "communication at all costs"'. For such work he was awarded the French Croix de Guerre.
On the night of 28/29 September 1917 at Zonnebeke a working party of 400 men was on its way to bury cable when a heavy enemy barrage commenced. Sergeant O'Brien volunteered to go through it to meet the party. Much depended on their work that night as an attack was to begin next day. He passed through the barrage safely and conducted the cable and on his own initiative brought up supplies. The work was completed and when tested all seventy pairs of cable required for that part of the divisional communications system were found to be in good order.
He was awarded the Distinguished Conduct Medal, promoted warrant officer on 4 October and commissioned second lieutenant on 6 November. During the German offensive from March 1918 at Albert and Dernancourt, France, as divisional lines officer O'Brien personally supervised his sections and maintained communications through the extremely heavy barrages of 5 April. Again in the words of Major Fraser, 'the importance of good communication on this eventful day, which decided the fate of the Great German Onrush and the results which he showed for his efforts, brought this gallant officer the Military Cross'.
After the Armistice Lieutenant O'Brien married Christina Urquhart on 28 November 1918 at Glasgow, Scotland.' AWMM