The loss of Paul O'Connell to the shoulder injury he sustained in Paris has allowed O'Callaghan to make a rare start alongside veteran Malcolm O'Kelly.
Ireland boss Eddie O'Sullivan suggested earlier in the week that O'Callaghan may have been preferred to O'Kelly, the nation's most capped player, had O'Connell been fit.
When all three British & Irish Lions are fit, O'Sullivan faces a major selection headache and O'Callaghan knows he must perform against Wales if he wants to retain the jersey for Scotland two weeks later.
"The competition for places is brilliant because you can't afford to play poorly and you can't afford to rest on your laurels," he said.
"You have to be playing well. I have been given a chance through injury to play and I will just be looking to perform.
"We have always had the opinion in the second rows that the two men who are lucky to play know that the man sitting on the bench is really breathing down their necks.
"So there comes an awful lot of pressure when you are given your chance and you have to take it."
O'Connell's loss is major blow as he has been Ireland's form player in their two Six Nations matches to date, landing the man-of-the-match award against Italy.
The Munster hardman is expected to be back in time for Scotland and O'Callaghan talks with great respect of his provincial team-mate.
"Paul O'Connell has set the standard for second rows in this country. People are quite aware that he is something special," he said.
"When I have chatted about Paul I have never tried to put myself in the same bracket as him because I think it would be disrespectful. He is a world-class player and will be a big loss to us."
It is difficult to predict what Wales team will show up at Lansdowne Road following their recent upheaval but O'Callaghan expects them to be firing on all cylinders.
"The recent problems can work in two ways as they can galvanise a squad as well. Sometimes when you are pinned into a corner you can gel together and push on for each other," he said.
"From what I know the Welsh lads they are not the types to sit around and moan and groan. They get on with things. Certainly it was a disappointment to lose (coach) Mike Ruddock.
"He was a good man in any dealing we had with him. The squad he has left there are good enough to go on. The players have to step up now and they will have no problem doing that."