In 2000, he represented Taoiseach Bertie Ahern in defamation proceedings against a businessman, appearing with Rory Brady and Paul Gallagher, both of whom later served as Attorney General of Ireland.[4] He was made a senior counsel in 2008.[5] O'Callaghan also served as a legal adviser to Fianna Fáil from January 2011 to his election to the Dáil. In 2014, he co-edited a book titled, Law and Government: A Tribute to Rory Brady.[6]
On 19 May 2016, he was appointed as Fianna Fáil Spokesperson for Justice and Equality by party leader Micheál Martin. He drafted and secured cross-party support for his Parole Bill which was passed by the Oireachtas and became an Act.[10] It reformed the parole system giving victims of crime and their families the right to be heard during the parole process.
O'Callaghan also introduced a Judicial Appointments Commission Bill on 18 October 2016 that sought to amend the process by which judges were appointed. Although the Bill passed second stage in the Dáil,[11] it was not supported by the Fine Gael minority Government which instead pursued legislation proposed by the Minister for Transport, Shane Ross, though with changes.[12]
In November 2016, O'Callaghan met with some of those who had been sexually abused as boys by Bill Kenneally, Fianna Fáil tallyman and cousin of former Fianna Fáil TD Brendan Kenneally.[13][14] Brendan Kenneally had previously acknowledged that he had been aware of his cousin's crimes before his sentencing to 14 years imprisonment.[13] On 10 July 2018 the Government established a Commission of Investigation under Judge Barry Hickson to investigate the Kenneally allegations.[15]
At the 2020 general election, O'Callaghan was re-elected as a TD for the Dublin Bay South constituency.[16] In July 2020 O'Callaghan declined the position of Minister of State at the Department of Justice in the 32nd Government of Ireland offered to him by Taoiseach Micheál Martin, stating that he wished to remain on the backbenches, providing a voice in Fianna Fáil outside of government, while also making the party more attractive to younger voters.[17] In September 2020 O’Callaghan told RTÉ radio he was interested in becoming the leader of Fianna Fáil following Martin.[18]
O'Callaghan was appointed as the party's spokesman on justice matters by Martin on 17 December 2020.[19]
O'Callaghan was the Fianna Fáil Director of Elections for the 2021 Dublin Bay South by-election in which the party's candidate came in fifth position.[20]
He played rugby at a senior level, representing UCD, Cambridge University, London Irish, Wanderers, Leinster and Connacht. He was also capped for Ireland at under-21 level. He is a frequent cyclist, regularly cycling to Leinster House and advocates for the expansion of cycling infrastructure in Dublin.