He was on the bench back in 2000 when Munster played Northampton Saints in that Twickenham final but at that stage of his career felt as much a spectator as anyone in the stand.
"I was just new in the squad then," he says, "so much in awe of lads like Gallimh, Claw, John Langford, KK (Killian Keane), Axel," and he laughs, "even Rog and he was only in a year ahead of me."
Mick Galwey and John Langford (legends he says) were the locks that day and two years later in Cardiff Galwey was partnered by Paul O'Connell with Mick O'Driscoll the back up.
But O'Callaghan was prepared to bide his time, learn his trade and when Munster were beaten 13-12 by Toulouse in 2003 he was O'Connell's partner. And again the following year in the semi-final defeat by London Wasps in Lansdowne Road when he was yellow carded- wrongly -and honestly admits, "I felt I left everyone down that day. My buddies, my family, the supporters."
In fact he rarely leaves anyone down and his dedication and professionalism has seen him achieve everything the game has to offer, Magners League honours, two Heineken Cup medals, several Triple Crowns, a Grand Slam and captaincy of the Lions.
But even more than that. He's earned the respect of colleagues and opponents.
On Saturday he'll set about his business in much the same way as he does for every game. "This competition is special. It's something that's very close to Munster supporters and we know that. Maybe its because at the start off we didn't do so well and people thought sure God love them. Then we won a few games, got to a final and just wouldn't let go. And our supporters bought into it. They're brilliant. and thats why" - and it's a recurring themewith him - "we feel we left them down last year. And last weekend as well."
So on Saturday the chance to start to make amends ?
"Northampton are a smashing side. Not unlike us. A proud provincial side. Great support. Great tradition. A good footballing side. And they dont lose too often at Franklin's Gardens. We're coming off a disappointing result last weekend. We know what we have to do. We've worked hard this week and hopefully we'll give a good account of ourselves."
Munster: P Warwick; D Howlett, L Mafi, J de Villiers, K Earls; R O'Gara, T O'Leary; M Horan, J Flannery, T Buckley; D O'Callaghan, P O'Connell capt; A Quinlan, D Wallace, D Leamy. Replacements: D Fogarty, J Brugnaut, Darragh Hurley, M O'Driscoll, N Ronan, D Ryan, P Stringer I Dowling.