Birmingham City: Alex McLeish insists Blues are back into top-flight swing
O'Connor, who had also been ill with a stomach bug during the week, settled the game when he came off the bench just after the hour and headed home from a 75th-minute corner.
It was a result that should put the club in good heart as they prepare for a change of ownership.
And Blues could have won more convincingly had Hull goalkeeper Boaz Myhill not produced a fine performance.
The Wales international was on top form and made several good saves including three from Christian Benitez, who was making his full debut, and two from Lee Bowyer.
"I thought it was going to be another hard-luck story but we still showed dedication and application and made the chances," added McLeish.
Yet it was not all Birmingham, and visiting goalkeeper Joe Hart also had a busy afternoon as mistakes from both sides allowed for plenty of goalmouth incident.
Hull had their fair share of chances and went close late on when Ibrahima Sonko headed against the bar in a penalty area scramble. Stephen Hunt also felt he was denied a penalty after a challenge by Scott Dann.
But time ran out for Hull and the defeat left them in the bottom three.
The Tigers have won just two of their last 28 games in the Premier League and they have done little to discredit the widespread pre-season belief they are destined for relegation.
Myhill admitted that losing to a promoted side in Birmingham was a heavy blow.
Hull now face a daunting trip to Liverpool next week and then have crucial games against Wigan, Fulham, Portsmouth and Burnley in October.
"It was ultimately disappointing," Myhill said. "It was a good chance for us to get some points and we haven't really delivered.
"We have got a month where we will have similar opportunities and we have got to deliver.
"We must improve and we must get some more points on the board because these are the kind of games we can't afford to lose.
"We know it was a game we needed to take at least a point from and unfortunately couldn't get it."
