[2] On 3 January 1902 he was promoted to captain,[3] and temporary appointed adjutant to the 2nd Life Guards.

[4] O'Neill fought in the First World War as a captain in "A" Squadron 2nd Life Guards.

[6][7] O'Neill is one of 19 MPs who fell in the war who are commemorated by heraldic shields in the Commons Chamber.

[8] A further act of commemoration came with the unveiling in 1932 of a manuscript-style illuminated book of remembrance for the House of Commons, which includes a short biographical account of the life and death of O'Neill.

[9][10] He was elected to the House of Commons for Mid-Antrim in January 1910, succeeding his uncle Robert Torrens O'Neill.