O'Sullivan oversaw the equipping of the contingents for the South Africa and First World Wars, and was responsible for seeing the Defence Stores Department through a period of modernisation and change.

He initially spent a year prospecting for gold on the West Coast, before moving to Wellington and joining the Native Department.

[2] O'Sullivan's duties in those strenuous times included the equipping of every contingent (all mounted men) for South Africa, attending to the volunteer camps and rifle meetings throughout the country.

On 8 April 1916, Captain Thomas McCristell, the Trentham Camp Quartermaster, was appointed to succeed Major O'Sullivan as Director of Equipment and Ordnance Stores.

[6] A son, Leo O'Sullivan, who was serving as a second lieutenant in the 1st Battalion of the Wellington Infantry Regiment in France, died of wounds on 24 August 1918.