After the Burning of Cork in December 1920, O'Callaghan who had received death threats, fled to America as a stowaway on board the steamship, West Cannon.[2] After being discovered by the Master of the ship, he was put to work as a crew member. He was arrested on arrival in the US but eventually freed, and spent the next eight months there where delivered a series of speeches and helped to secure a loan for Dáil Éireann, acting as the emissary of Michael Collins.[4] O'Callaghan returned to Cork in August 1921.
On 25 January 1924, he resigned his position as Lord Mayor of Cork. Later in 1924, he married Eibhlín Ní Shuilleabháin in London. They lived in London and later in Strasbourg before returning to Ireland in 1929. O'Callaghan secured a job as an accountant for the ESB.[2] He died in Dublin in 1962, and is buried in Deans Grange Cemetery, Blackrock, Dublin.[2]