O’Callaghan, a development contracted player from Youghal, comes into the side as the 2006 and 2008 champions face a injury crisis in the position.
With back-rowers Niall Ronan, Sean Dougal, Tommy O’Donnell all on the injured list, opportunity knocks for O’Callaghan who has been handed the number 6 shirt.
O’Callaghan made his Heineken Cup debut in the final pool game last season and has been a regular in this year’s RaboDirect Pro12 matches.
There are four other changes from the team that lined up against Edinburgh last time out.
James Coughlan, Ronan O’Gara and Keith Earls return from injury while Felix Jones is promoted from the substitute’s bench.
The game at Thomond Park start at 6pm.
Replacements: D Varley, W du Preez, S Archer, B Holland, P Butler, D Williams, I Keatley, C Laulala.