But there was no holding back by the players on duty against Neil Warnock's Crystal Palace as two late John Carew penalties secured a 3-1 success in the fifth-round replay - and an away clash with Reading.
Tottenham Transfer TalkAston Villa keep FA Cup high on their agendaO'Neill, who fielded close to a full strength line-up, said: "I'm naturally delighted for the club but more genuinely pleased for the players.
"All the players were prepared to go for it and I didn't see one shirking anything at all in preparation for Sunday. That speaks volumes for the team.
"I am really pleased with the team. It is showing some fantastic ability and they are improving which is great. They have great commitment.
"We have lots of things to improve on - but the commitment is there in abundance.
"We went very strongly, only made a couple of changes and, I think the way Palace played in the second half, showed we had to do that.
"But we deserved to win the game although we left it very late.
"We spurned a lot of chances and it was one of those occasions where you definitely needed the second goal.
"I thought it might be one of those evenings when Palace equalised but we turned it around and deservedly went through."
O'Neill has plenty of respect for Reading, who won 3-2 at West Brom, but will now turn his thoughts to the meeting with Sir Alex Ferguson's side.
He said: "Reading have had a wonderful FA Cup. It doesn't seem to matter to them where they go. They seem to be able to win away from home.
"But, for the first time, I will be able to concentrate now on the Carling Cup final."
O'Neill will check on the fitness of a couple of his players today after they picked up knocks against Palace.
He said: "There were one or two in the dressing room and I just stayed in there a little bit longer to see how they were. We will have a better idea of them in the morning."
Warnock will still be in charge of Palace for Saturday's Championship clash at Doncaster but refused to deny speculation that a move to QPR may happen in the near future.
He said: "Doncaster beat us 3-0 at our place so I've said to the lads we need to rest now, have got a long journey again on Friday, have got to do things right.
"It is a major game for us. I don't like being in the position we are in (the bottom three) and we shouldn't be in the position we are in but football is tough at times.
"Avoiding relegation is now the priority. There is nothing else left in the season.
"Can I deny the speculation that I are moving to QPR - No. Could this have been my last game? It could have been my last game - but it won't be."
FA Cup quarter-final draw
Chelsea v Stoke CityFulham v Tottenham HotspurReading v Aston VillaPortsmouth v Birmingham City