Was educated at Queen's College Galway. Graduated MD in 1881 and MCh in 1882 at the old Royal University of Ireland.
2 Feb 1884 Gazetted Surgeon.
Apr 1889 Served in the Bombay Command. Was granted leave of absence for private affairs to England for six months.
Dec 1889 On returning from furlough, was posted to general duties to Aden district.
2 Feb 1896 Promoted Surgeon-Major.
Malta 7 Oct 1896 Arrived from England.
Malta Jan 1897 Officer in charge of Staff and Dependents Valletta.
Embarking and disembarking medical officer.
Resident at Great Britain Hotel Valletta.
Malta 22 Mar 1897 Embarked for Crete where he was in charge of the Field Hospital at Candia.
Malta 31 Aug 1898 Returned to Malta.
Malta 16 Oct 1898 Home on sick leave.
Malta 26 Apr 1899 Returned to Malta.
Malta Jan 1900 Arrived from Crete.
Malta 29 May 1900 Returned to Malta.
Malta 6 Dec 1900 Home on leave.
Malta 27 Dec 1900 Returned to Malta.
Malta 1901 On garrison duty.
Malta 30 May 1902 Present at a General Meeting of the Malta and Mediterranean Branch of the British Medical Association which was held at Valletta under the presidency of Surgeon-General Thomas O'Farrell.
Malta 3 July 1902 Home leave.
Malta 5 Sept 1902 Returned to Malta.
Malta 20 July 1903 Returned to England to act as the delegate representing the Malta and Mediterranean Branch, at the seventy-first annual meeting of the British Medical Association at Swansea on 28 July 1903.
Malta Aug 1903 Passed his examinations for promotion to Lt Col. Passed in the following subjects: Army Medical Organisation, Sanitation and Epidemiology, the Special Subject, and Military Law.
Malta 2 Feb 1904 Lieutenant Colonel RAMC.
13 Feb 1913 Retired.
28 July 1914 Start of the Great War.
4 Aug 1914 Britain declares war on Germany.
6 Nov 1914 Recalled during the Great War.
3 June 1917 Brevet Colonel.
16 Apr 1919 Retired again.
23 Nov 1920 Appointed to the retired pay at Bury St Edmunds.
Col O'Connell had married the granddaughter of Inspector General James Forbes MD, founder of the RAMC HQ Mess, who served in the retreat of Corunna. His son Lt John Forbes O'Connell RAMC was killed in action at the battle of the Aisne in 1914.