"There's no fear in the squad. We've a huge task at hand, we're well aware of that. If you look at Argentina, they haven't conceded a try yet in the tournament and we have to go out and score four against them on Sunday.
"It's a chance to go out and put in a big performance, the fellas are down now but we've got to pick ourselves up and go out and deliver next weekend because this is something we've trained hugely hard for."
Giving his opinion on Ireland's lacklustre displays in Pool D to date, O'Callaghan was quick to take the blame away from the coaching staff.
"It comes down to the players. We haven't been performing as well as we need to. To reach the pinnacle of your career and not produce the performance you know you're capable of is very disappointing. We almost feel like apologising for our performances. Nobody goes out to play poorly."
The press allegations about out-half Ronan O'Gara's personal life in the lead-up to last Friday's France match were "a nuisance and unfair and something the team didn't need at this time", O'Callaghan conceded.
"What's going on is terrible but it sums up Rog (Ronan O'Gara) and how much of a professional he is. If I was in his position, I don't think I could have been as mentally strong as he's been all week.
â00Having to put up with that bull**** is annoying. But I believe he will still be able to build himself up for the Pumas."
Meanwhile, Ireland's only injury worry to emerge from that 25-3 defeat to the French is full-back Girvan Dempsey who damaged his shoulder when trying to stop Vincent Clerc from scoring his second try at the Stade de France.
Commenting on the squad's fitness, coach Eddie O'Sullivan said: "Girvan is our only doubt, the rest of the players are fine and we will announce the team to face Argentina on Tuesday."