"This is our most important game. We've always had this match at the back of our minds. Coming into the world cup we knew this was the one we needed to win."
"We let down our supporters in 1999 but this time that won't happen. We're going out to make amends for it. It's time to alter that blip in our careers," he said.
O'Brien said that Ireland had been building up to this game since August and the team were physically and mentally prepared for it.
"We're under no illusions as to how difficult it is going to be. It's going to be a ferocious game, there will be nothing won without a fight, but we are prepared for that.
"We have done our homework, we have a game plan. We know what they are capable of and we're better. I think it will go right to the wire, but we have the strength and the depth in our squad to beat them."
O'Brien dismissed criticism of the Irish pack from Namibian coach David Waterston last weekend, saying they were "just as good, if not better" than their Argentine counterparts.
He also said the three-week start to the tour had been brilliant, adding the team settled in very well in Terrigal. "It was a good choice, it wasn't be accident we based ourselves there, we had first hand information about it," he said.
"We had some experience of Terrigal and myself and Eddie O'Sullivan came to the Central Coast after the New Zealand test series last year.
"We got in early, there were others looking for it as a base," he revealled.
On the injury front, he said that prop Reggie Corrigan's shoulder muscle injury was healing well. Gary Longwell's calf injury was taking longer to heal however.
The Irish manager also paid tribute to the loyal Irish fans, predicting they would "travel to the moon" to support the Irish team.
"The Irish support has been exceptionally good, all the team are talking about it. The Irish have really shown their colours. We are delighted with the support, it makes a big difference and I hope it continues," he added.
Irish skipper Keith Wood has also put his weight behind an Irish victory. "I think we have grown considerably as a team since then [1999 World Cup], our management and preparation is much better," Wood said.
"We had a poor game plan that day and we weren't good enough. Argentina did play very well that day, but the manner in which we played was very disappointing."
Ireland plays Argentina at 7.30am Irish time on Sunday. A win will put Ireland into the quarter-finals, whereas a loss would mean Ireland would need to beat Australia to make the last eight.
In the five games played between Argentina and Ireland, the Pumas have won two of the past three, with Ireland winning the last clash 16-7 at Lansdowne Road, Dublin, in November last year.