[2] He married Hélène Leopoldine Francoise Isabelle De Witte at St Patrick's Cathedral, Auckland, on 4 February 1856, with the wedding ceremony carried out by Bishop Pompallier.
His Belgian-born wife was from Waiheke Island, and her father Charles Antoine Joseph De Witte was the Belgian consul to New Zealand.
He was one of the first two solicitors to be submitted to the Supreme Court in 1851 after having trained in New Zealand.
[1] O'Brien represented the City of Auckland electorate in the 1st New Zealand Parliament from 1853 to 1855, when he retired.
[8] He died at Woodside Private Hospital in Burleigh Street, Auckland, on 17 April 1901 following a long illness.