But O'Callaghan says that joking around will be the last thing on his mind when he leads the Lions into action against the Southern Kings in Port Elizabeth this afternoon.
"I'm known as a bit of a practical joker, but when it comes to playing rugby there is a different side to me," said O'Callaghan, after being named skipper of a side that includes seven other Magners League players in the starting XV and a further five among the replacements.
"This means an awful lot to me. Geech (Lions head coach Ian McGeechan) doesn't mess around so I didn't think it was a practical joke from him! He said that nothing was sorted for Saturday, but he'd like me to lead the side.
"It s a huge honour and it will be the highlight of my rugby career."
O'Callaghan is currently on his second tour with the Lions having travelled to New Zealand as a 26-year-old in 2005. After missing out on selection for the first Test four years ago, O'Callaghan kept his head high and went on to feature in both the second and third internationals against the All Blacks.
He hopes that he will be given the nod from the start for the forthcoming series against the Springboks - an attitude he believes will be mirrored by each and every one of his team-mates later today.
"The guys are going out to leave the coaches in no doubt they want to make the Test team," he added.
"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."
Ospreys wing Shane Williams has recovered from a mild stomach bug but is only considered fit enough to take a place on the bench at the Nelson Mandela Bay Stadium.
Ugo Monye is promoted to the starting line up but Williams' inclusion as a replacement means Tommy Bowe will not be required in the matchday 22.
British & Irish Lions: Keith Earls; Ugo Monye, Riki Flutey, Gordon D'Arcy, Luke Fitzgerald; Andrew Sheridan, Ross Ford, Euan Murray, Donncha O'Callaghan (captain), Simon Shaw, Nathan Hines, Joe Worsley, Andy Powell
Replacements: Matthew Rees, Adam Jones, Tom Croft, David Wallace, Harry Ellis, Ronan O'Gara, Shane Williams
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