O’Conor is eldest son in the Roman Catholic family of Denis O’Conor of Bellanagare and Clonallis, County Roscommon, and Mary, daughter of Major Blake of Towerhill, County Mayo. A younger brother, Denis Maurice O’Conor (1840-1883), is a Liberal Party MP in the Home Rule interest for Sligo County (1868-83).
After his education at Downside School in England, O’Conor enters London University in 1855, but does not graduate. He enters public life at an early age, being elected MP for Roscommon as a Liberal Party candidate at a by-election in 1860. In 1874 he is returned as a home ruler but, refusing to take the party pledge exacted by Charles Stewart Parnell, is ousted by Irish nationalist journalist James O’Kelly in 1880. In 1883, he is defeated by William Redmond in a contest for MP representing Wexford Borough.
An active member of parliament, O’Conor is an effective though not an eloquent speaker and a leading exponent of Roman Catholic opinion. He frequently speaks on Irish education and land tenure. He criticises unfavourably the Queen’s Colleges established in 1845 and the model schools, and advocates separate education for Roman Catholics. In 1867 he introduces a measure to extend the Industrial Schools Act to Ireland, which becomes law the following year.
O’Conor opposes William Ewart Gladstone‘s Irish University Bill of 1873, and in May 1879 brings forward a measure, which has the support of almost every section of Irish political opinion, for the creation of a new examining university, St. Patrick’s, with power to make grants based on the results of examination to students of denominational colleges affiliated to it. This is withdrawn on July 23 on the announcement of the University Education (Ireland) Act 1879 creating the Royal University of Ireland.
O’Conor steadily lurges a reform of the Irish land laws. On social and industrial questions he also speaks with authority. From 1872 onwards he professes his adherence to home rule and supports Isaac Butt in his motion for inquiry into the parliamentary relations of Great Britain and Ireland in 1874. He also acts with the Irish leader in his endeavours to mitigate the severity of coercive legislation, though declaring himself not in all circumstances opposed to exceptional laws.
Following his parliamentary career in 1880, O’Conor is a member of the Registration of Deeds Commission of 1880, and takes an active part in the Bessborough land commission of the same year. He is a member of both the parliamentary committee of 1885 and the royal commission of 1894 on the financial relations between Great Britain and Ireland, and becomes chairman of the commission on the death of Hugh Culling Eardley Childers in 1896. He is also active in local government, presiding over parliamentary committees on Irish grand jury laws and land valuation in 1868 and 1869, and being elected to the first county council of Roscommon in 1898. He is Lord-Lieutenant of the county from 1888 until his death.
O’Conor is much interested in antiquarian studies. He serves for many years as president of the Antiquarian Society of Ireland, as well as of the Royal Irish Academy. He is president of the Irish Language Society, and procures the insertion of Irish language into the curriculum of the intermediate education board.