Throughout his career, O'Callaghan played primarily as a lock, though he occasionally provided cover at blindside flanker.

The following year he was part of the U-19 World Championship winning team, along with Brian O'Driscoll and Paddy Wallace.

[8][9] O'Callaghan scored a try after a 55m run in the first game of their Heineken Cup defence against Leicester Tigers at Welford Road Stadium.

[16] O'Callaghan became the ninth Munster player to reach the 200-caps mark in the league game against Glasgow Warriors on 14 April 2012.

[24] On 5 December 2015, O'Callaghan received the first red card of his career during Worcester's game against Leicester Tigers.

In January 2017, O'Callaghan captained with Worcester for the first during their 2016–17 European Rugby Challenge Cup fixture against Russian side Enisei-STM.

[26][27] O'Callaghan made his international debut for Ireland on 22 March 2003, coming on as a replacement against Wales at the Millennium Stadium during the 2003 Six Nations Championship.

[38][39] O'Callaghan was part of the Irish team which won the Grand Slam in the 2009 Six Nations Championship, Ireland's first in 61 years.

[78] O'Callaghan was named in Ireland's training squad for the 2013 Six Nations Championship on 17 January 2013,[79] and came off the bench against Wales[80] and England.

[81] An injury to Mike McCarthy meant that O'Callaghan was promoted to the starting XV for Ireland's game against Scotland on 24 February 2013.

[83] O'Callaghan was then dropped from the international side, with Leinster's Devin Toner being preferred for the place on the bench against Italy.

O'Callaghan was selected in Sir Clive Woodward's squad for the 2005 British & Irish Lions tour to New Zealand on 11 April 2005.

[89] O'Callaghan was also selected in Sir Ian McGeechan's squad for the 2009 British & Irish Lions tour to South Africa.

On 23 December 2009, O'Callaghan married his longtime girlfriend, Jennifer Harte, at St. Columba's Church in Douglas, Cork.

[105] In May 2021, he was announced as one of the new presenters of The RTÉ 2fm Breakfast Show alongside Doireann Garrihy and Carl Mullan, which began on 31 May.