But the coach told a press conference: "If our play is similar to last week's game, I'd be happy."
Flanker Shane Jennings and wing Rob Kearney will make their senior debuts on Saturday at Jose Amalfitani stadium.
And O'Sullivan said: "We defended very well in the first Test, our set piece worked well, but we gave Argentina opportunities to turnover football, and they are very good at that.
"It was a very physical Pumas side and I think it will be the same next weekend.
"I think we have to play as we did on Saturday and better in terms of execution and discipline.
"We have a pretty unique situation as we are playing two games over two weeks.
"If you look at the Irish team, it's a different team to last week. There are 11 personnel changes and one positional switch."
He added: "I'm not sure what Marcelo [Loffreda] will do with his side, which changes he will make, but I think he is in a similar position as us.
"This is an unusual game because some players have only one week to be prepared. From my point of view, what happens on Saturday is quite unpredictable.
"It's a different game, a different situation. I would have been very happy if we had won last Saturday, and we could have won. And I want to win this weekend, so I hopefully will."
Jeremy Staunton has arrived in Buenos Aires as a replacement for knee injury victim Paddy Wallace, while Andrew Trimble and Tommy Bowe have also been ruled out.
Eoin Reddan, a European Cup winner with Wasps, will win his second cap and take the scrum-half place filled by Isaac Boss in Santa Fe.
O'Sullivan added: "A scrum-half has to be like a boss on the pitch.
"He needs to manage the pack and give a good service and kick well... and Eoin does all this very well.
"He is like a complete pack for Saturday's game, because he is a good passer, he kicks well. He played very well for Wasps and we hope he can show his potential with Ireland as well."
After scoring Ireland's only try in Saturday's defeat, Brian Carney will be on the wing, having a new chance to impress O'Sullivan, with the focus on the upcoming World Cup in France.