Thomas J. A. O'Callaghan entered the military service in Massachusetts. He served in the U.S. Army during World War I as a Corporal in the 103rd Infantry Regiment, 26th Infantry "Yankee" Division.

The following letter from Second Lieutenant F. J. Ellis, of Co. B, 103 d U. S. infantry,has been received by Mrs. O'Callaghan, mother of Corporal Thomas O'Callaghan, who died from wounds received in France, giving the particulars of his death: September, 27. 1918. My Dear Mrs. O'Callaghan:—This is the first opportunity I've had to write to you since your son's noble death, for shortly after he was wounded I was wounded myself, and am just able to be around now. Your son Tom was one of my corporals. I first met him in Westfield, and the longer I was associated with him the more I admired him and we soon grew to be firm friends. His cheery smile was never clouded, and his good humor and fund of stories seemed to have no end. Since he was a bit older than most of the men from the platoon, he was often consulted by them in their troubles, and he always had a patient ear and a word of advice for them. He was loved by the men as a true soldier and comrade, and all the officers that came to know him admired him and respected him. His end came through his willingness to sacrifice himself for others. In July we were bivouaced in a woods. One night we were shelled heavily. In the hole which was shared by Tom and his bunkie, another soldier crawled for safety. The hole was not large enough for three, so Tom forced the two youngsters to remain safely in his hole, and he crawled towards a nearby shelter, thirty yards away. While on the way a piece of high explosive struck him, wounding him seriously in the stomach. He died a few days later in the hospital. After giving him first aid and placing him on the stretcher he was fully conscious, and seemed lo know that his wound was fatal. His death was an act of self-sacrifice, a truly noble end. He lived a good Catholic life while over here, and I am sure that even now he is enjoying the peace of Heaven In union with Our Divine Lord and His Blessed Mother, the peace and happiness he so richly deserves. I am proud to have been associated with him. and his heroic self-sacrifice shall always he an example to me. While I sympathize with you, his mother and the members of the family in your grief, yet with you, I shall feel proud to have known Tom, to have known him a true soldier and a brave Christian, who by his example taught many of us how to act when in the face of danger. I shall continue to remember your son in my prayers, and I pray that Our Divine Saviour may comfort you in your sorrow. Your son followed His example, in that he laid down his life that others might live. If I ever return to Boston, I shall surely call on you, for I am very anxious to meet the mother of such a noble, brave, true soldier as was your son Tom. Sincerely, Frederick J. Ellis. 2nd Lieut. Co. B. 103 rd US Inf.
Source of information: Cambridge Chronicle, 2 November 1918, www.findagrave.com, www.abmc.gov