O'Connor came off the bench to settle a scrappy Barclays Premier League contest at the KC Stadium by heading home after 75 minutes. Injuries had forced McLeish to name just six substitutes for the game and it could have been worse had O'Connor not recovered from a sickness bug.
"We have had a catalogue of injuries that would absolutely frighten you. We were down to the bare bones. Garry was on his sickbed as well. When he came on you wouldn't have known he had been ill, he was very sharp."
Birmingham had the better opportunities throughout the game but were frustrated by the performance of Hull goalkeeper Boaz Myhill.
Myhill produced a number of good saves to deny the visitors, with Christian Benitez and Lee Bowyer thwarted several times between them. Ecuador striker Benitez, who sported a bandage after an early clash of heads that required stitches, looked particularly lively on his first league start.
"I thought the wee man came alive when he got a head knock," McLeish added. "He was willing to do his bit and became very dangerous in the game."
Hull manager Phil Brown put his side's defeat down to a lack of concentration and admitted, with just four points from six games, confidence is low.
"Birmingham put in a shift, a performance worthy of frustrating the home opposition," said Brown.
"For someone to walk into our six-yard box and get a free header needs to be looked at. Confidence levels are now low and they need picking up."
Brown added that he is hopeful midfielder Geovanni will soon commit to a new contract.