| Regimental number | 24291 |
| Date of birth | |
| Place of birth | Lincoln, North Canterbury |
| Religion | Church of England |
| Occupation | Farmer |
| Marital status | Married |
| Age at embarkation | 37.2 |
| Height | 5' 11" |
| Weight | 124 lbs |
| Next of kin | Wife, Mrs J M S O'Callaghan, c/o Bank of New Zealand, London, England |
| Previous military service | Served in the 2nd and 8th New Zealand Contingents in South Africa (Boer War). Major, General Staff, Canterbury Military District, advised HQ NZ Military Forces, 16 February 1914: 'He [O'Callaghan] served with the Cadets for 2 years, in neither of which he was efficient.' Colonel R.A. Chaffey, Commanding Canterbury Military District, wrote to Headquarters, NZ Military Forces, 1 April 1916: There is no one at present in the District who can recommend this gentleman or who knows anything about his capabilities as an officer.' Not serving in any military unit at time of NZEF enlistment. |
| Enlistment date | |
| Rank on embarkation | Captain |
| Unit name | New Zealand Expeditionary Force, 18th Reinforcements, Canterbury Infantry Battalion, C Company |
| Embarkation details | Unit embarked from Wellington on board HMNZT 67 Tofua on |
| Final rank | Captain |
| Final unit | 1st Battalion, Canterbury Infantry Regiment |
| Fate | Killed in Action |
| Age at death from cemetery records | 38 |
| Place of burial | No known grave |
| Commemoration details | Tyne Cot Memorial (New Zealand Apse, Panel No 2), nr Ieper, Belgium |
| Miscellaneous information from cemetery records | Parents: Arthur Pyne O'Callaghan and Florence O'Callaghan, 16, Craigie Street, Timaru; Wife: Julia Marie O'Callaghan, of Hadlow, Timaru. Native of The Springs, Lincoln, Canterbury. |
| Other details |
War service: Western Front Embarked Wellington, 11 October 1916; disembarked Plymouth, England, 29 December 1916, and marched into Sling Camp. Proceeded overseas to France, 1 February 1917; joined 1st Bn, Canterbury Infantry Regiment, in the field, 14 February 1917. Detached to 2nd Army School, 21 June 1917; rejoined unit, 27 July 1917. On leave to England, 19 August 1917; rejoined unit from leave, 7 September 1917. Killed in action, 12 October 1917. Buried Passchendaele Waterloo Farm; grave subsequently lost. Medals: British War Medal, Victory Medal |
| Sources | Nominal Roll Vol. 2: Roll 43, p. 9 Archives New Zealand (Wellington), Agency AABK, Series 18805, File 20524691 |