[5] He made his Heineken Cup debut against Northampton Saints on 21 January 2012, in Round 6 of the pool stage.

[8] On 27 April 2012, O'Callaghan started for the Munster A team that beat Cross Keys 31–12 to win the final of the 2011–12 British and Irish Cup.

[12] O'Callaghan started in Munster's opening game of the 2015–16 European Rugby Champions Cup on 14 November 2015.

[15] He made his return from a knee injury on 19 January 2018, coming off the bench for Munster A in their 27–0 British and Irish Cup win against Ospreys Premiership Select.

[17] O'Callaghan left Munster to join French Rugby Pro D2 side Biarritz ahead of the 2019–20 season.