"It s a huge honour and it means so much to me. It will be the highlight of my rugby career," said O'Callaghan.
"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."
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 on Tuesday afternoon.
"The guys are going out to leave the coaches in no doubt they want to make the Test team," added O'Callaghan.
"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."
Simon Shaw will pack down with O'Callaghan in the second row with a third lock, Scotland's Nathan Hines, featuring in the back row. Leinster's Luke Fitzgerald gets the chance to push his claims for a wing berth against the Springboks with another start out wide, while Munster's Keith Earls is switched to full back.
For an in-depth match preview of the Lions versus the Southern Kings, simply click here