Wolves led 1-0 after half an hour of the fourth round clash with Roberto Mancini's side on Wednesday night but were destroyed as City scored five times in 27 minutes either side of half-time. Both sides will likely make wholesale changes for the league meeting but O'Hara knows City's strength in depth means Wolves cannot afford to make the same sort of errors.
He said: "The boys had a good start the other night and we were on top of them. But with teams like Manchester City, you give them one goal, they sense you are on the ropes, and they punish you."
O'Hara added: "That's what City do. They've done it against Manchester United, put six past them, and United are no mugs.
"They have got great players in abundance. They are ruthless. They will let you play but give the ball away, or don't follow a man, and they will punish you.
"That is something we've got to be aware of on Saturday and we'll be working on that.
"But the scoreline was flattering to them. We created chances and played some good stuff and we want to take that into Saturday. We will probably have to defend for our lives and hope we can nick something from it."
O'Hara revealed he has been given the go-ahead to get forward more by Wolves boss Mick McCarthy with the result being goals in his last two games.
O'Hara said: "I had a chat with the gaffer and he is going to try and play me a bit further up the field, maybe try and get me in the hole.
"It's not a free role because you've got to pick up the sitting midfielder. But when we've got the ball you can get into attacking positions and for me, that's where I think I'm most effective, trying to create rather than hitting balls from the halfway line."
Source: PA
Source: PA