The big Corkman may have a reputation for being a practical joker off the pitch but he is deadly serious about matters on it and is determined to maintain the Lions' unbeaten record on the 2009 tour in what will be his first game as a captain since his schooldays.
His tenth appearance for the Lions will hopefully be one to remember and with 144 Munster caps, 55 Ireland caps and Heineken Cup, Magners League, Triple Crown and Grand Slam honours behind him, O'Callaghan will use his immense big-game experience to lead from the front.
"It is the biggest honour I have ever had bestowed on me, and you must enjoy moments like this when you're living out your dreams," he said, looking forward to the Port Elizabeth fixture.
"It doesn't get much better than this. I was completely taken aback when 'Geech' (Ian McGeechan) broke the news to me.
"For once, I was speechless. It's fantastic, really brilliant. It will be the highlight of my rugby career.
"Geech doesn't mess around so I didn't think it was a practical joke. He said that nothing was sorted for Saturday, but he'd like me to lead the side.
"I'm known as a bit of a practical joker, but when it comes to playing rugby there is a different side to me. It means an awful lot to me."
As well as taking on the responsibility of the captaincy role, O'Callaghan has the aim of putting in a big individual performance - like every member of Tuesday's matchday squad - with the Lions management due to sit down and select the team for the first Test on Wednesday.
Many observers will argue that the Test side has already been selected, with the front runners for those prized starting berths already decamping to Durban. Tour captain Paul O'Connell, Brian O'Driscoll and Jamie Heaslip are amongst that number.
But McGeechan has told the O'Callaghan-led squad that they are still fighting for Test selection and reminded them of how Ulster and Ireland lock Jeremy Davidson achieved the feat of playing against the Junior Springboks on Tuesday and then starting the first Test the following Saturday on the 1997 tour.
"I told the players that I won't pick the Test team until after the final warm-up game and I won't change that," McGeechan said at the team announcement press conference.
"I would not have been honest with the players if I had not done that. This is Test week and there is a noticeable change in attitude within the touring party.
"We are now heading towards the business end of the tour and the reason why we have come to South Africa.
"As you would expect, the immediate focus is the Southern Kings before attention turns to selection for the first Test, for which all players will be considered."
His South African counterpart, Peter de Villiers, has publicly stated that he reckons that the Lions Test team was chosen some time ago and that the side that started last week's game against the Sharks was effectively the Test selection, bar one or two changes.
Having played against the All Blacks on the ill-fated 2005 tour, O'Callaghan would dearly love to be part of a Test series-winning side in South Africa and he realises the importance of Tuesday afternoon with the three Tests to follow and an intriguing game against the Emerging Springboks.
"The guys are going out to leave the coaches in no doubt they want to make the Test team," added the 30-year-old.
"There is nothing really to captaining a side like this. I won't be telling them what to do because they are hugely motivated. That will make it very easy for me to captain the side.
"We are a group of players who get on really well and this tour has got the feel of a club side.
"The Southern Kings have a lot of good players in their side and its going to be another tough game, but we'll be looking to take the hand-brake off and worry about ourselves rather than them."