It was the day, in October 1978, when Munster beat New Zealand 12-0 and players like try-scorer Christy Cantillon carved a permanent place for themselves in Irish rugby folklore.
Now, it is lock O'Callaghan's turn under the spotlight, lining up against the All Blacks for his British & Irish Lions Test debut on Saturday when he will be partnered by Munster second-row colleague Paul O'Connell.
"It is a day that is always talked about in Munster's history," said the 26-year-old Irish international.
"It was before I was born but I could almost tell you the team, with Christy Cantillon scoring the try. It is held in highest regard as the greatest Munster victory.
"For a Munsterman to go out there on Saturday - and to have Paul (O'Connell) with me as well - is a great honour, and one I am very proud of.
"Playing against the All Blacks is something you dream of, and in a Lions jersey too, it couldn't be any more special.
"When you talk about big games, you talk about New Zealand. Growing up, all your heroes would have been Kiwis, coming from where we do, and to get this chance is incredible."
Cork-born O'Callaghan is one of seven new faces in the Lions side following last week's 21-3 first Test defeat and he has a key job as his side look to establish the upper hand in the lineout.
"The main thing is we don't need to panic," he added. "We are all experienced enough to calm things down, trust each other and make sure things go right.
"I am sure that if the lads who played last week were playing again, there would be a drastic improvement in the lineout anyway."
The combination with O'Connell is crucial to Lions' victory hopes and O'Callaghan believes they will inspire each other.
"We both expect high standards of each other, and we know we have to raise the bar a little bit for this game," he said. "It is not like a club match or an Irish game we've played together.
"I am just very keen to play my own game, and get the basics right. I know there are huge things at stake, but I am not going to change the things I've always done."
O'Callaghan has also revealed the absence of injured Lions and Ireland captain Brian O'Driscoll will spur him on at Westpac Stadium.
O'Driscoll, whose right shoulder was dislocated after just 77 seconds of the first Test, will watch Saturday's showdown from a grandstand seat, rather than directing operations out in the middle.
"I am a huge fan and friend of Brian's, and I am just disappointed to see the fella's tour cut so short," added O'Callaghan.
"We really didn't get to see the best of him. I think the Irish lads will know his season has been teed up for this, and I think we have all lost out.
"It is a pity but when you lose a friend to injury like that, it's something that picks you up and makes you more determined."