He was the eldest son of Thomas O'Connor, an Athlone shopkeeper, and his wife Teresa (née Power), the daughter of a non-commissioned officer in the Connaught Rangers.

From 1867, O'Connor attempted unsuccessfully to gain a position in the Civil Service before working for the Royal Irish Constabulary as a reporting assistant on nationalist political demonstrations.

In 1870, he moved to London, and was appointed a sub-editor on The Daily Telegraph, principally on account of the utility of his mastery of French and German in reportage of the Franco-Prussian War.

O'Connor had been invited to stand as the Liberal candidate for the constituency on the strength of the unflattering biography of then Conservative Prime Minister Benjamin Disraeli he had written in 1878.

At the 1918 general election, following the retirement of Liberal-Labour MP Thomas Burt, O'Connor became "Father of the House of Commons", and ultimately would achieve unbroken service of 49 years, 215 days.

Having drawn the admiration of even the Ulster Unionist leader Sir Edward Carson, he played a leading role in the Irish Convention in 1917.

On 13 April 1920, O'Connor warned the House of Commons that the death on hunger strike of Thomas Ashe would galvanise opinion in Ireland and unite all Irishmen in opposition to British rule.

[5] He nonetheless supported the 1921 Anglo-Irish Treaty, although he subsequently appealed in vain to the British government to moderate its demand for a substantial contribution from the newly created Irish Free State to the imperial exchequer.

On his eightieth birthday in 1928, O'Connor was invited to dine with King George V, and despite being confined to a wheelchair, he continued to attend parliamentary debates until his death.

The couple, who had no children, rarely lived together, and had separated permanently by the outbreak of World War I, but remained married until O'Connor's death.