O'Connell is widely regarded as one of Ireland's leading players, and O'Callaghan knows stepping up the standards set by his Munster team-mate will require he pulls out all the stops.
"Paul O'Connell has set the standard for second rows in this country," he declared in an interview with the Irish Times. "People are quite aware that he's something special. When I've chatted about Paul, I've never tried to put myself in the same bracket as him because I think it would be disrespectful. He's a world-class player and will be a big loss to us come Sunday."
Still, O'Callaghan, who was pushing for inclusion even before O'Connell's withdrawal, is determined to make the most of his unexpected opportunity.
"Through injury I've been given a chance on Sunday and I'll just be looking to perform," he admitted. "We've always had the opinion within the squad, the secondrows, that the two men who are lucky to go in under pressure know that the man sitting on the bench is really breathing down their necks. So, there comes an awful lot of pressure when you're given your chance and you have to take it."