Liam Trotter had given the Lions an early lead before Billy Sharp levelled for the hosts midway through the first half.
And that was the way it remained until Darius Henderson, Chris Wood and Andy Keogh added further goals for the visitors during the final half-an-hour.
O'Driscoll said: "We didn't really see that coming - the game was quite tight and at half-time it was 50-50.
"It looked as though our front two or their front two would win and their front two did it for them.
"They came here with a six-game unbeaten run and you could see why. They get the ball up to the front two who are mobile and they play off that and in this division it's as good a tactic as you can have.
"They have a strong back four, legs in midfield and if they keep everyone fit they will be as good as anyone else in this league.
"We are still forming as a team and didn't function as well as we have been recently.
"There were reasons for that like injuries, suspension and enforced changes. We keep swapping and changing - it's not an excuse but it doesn't help."