[2][3] His military service was partially sidelined as his political career began, and he eventually retired in order to stand for election.

[5][6] In 1885 O'Neill stood in the by-election for the constituency of Antrim, following the death of James Chaine.

He was returned four times unopposed between 1886 and 1906, and successfully defeated John Hamilton Verschoyle in his final election in 1906.

[7] O'Neill, was heavily involved in a number of charitable institutions, and succeeded his father as president of the Royal Belfast Hospital for Sick Children in 1883.

He was never married and died on 25 July 1910 at his London residence, 11 St James's Square, Westminster, following a long illness.