Alex McLeish believes Birmingham fans seldom saw the best of O'Connor because of injury setbacks after his three-year spell at St Andrew's came to an end.
O'Connor has signed for Barnsley until the end of the season after two loans spells at Oakwell.
The Scotland international was given only a six-month contract by City last summer after undergoing operations on both hips.
That deal expired on Saturday and now O'Connor has made permanent his move to the Championship club.
McLeish said: "We wish Garry well and it's great for him that he has signed a deal with Barnsley.
"He is a good player and it is unfortunate Birmingham fans never got the chance to see the very best of him during his time here.
"They saw flashes of it with some of the performances he put in and important goals scored, particularly in the promotion season."
McLeish admitted: "The double hip injury Garry suffered was a real blow to him just when it looked his Birmingham career was on an upward curve.
"He is a great character and fighter and came back from that. We just felt - and I am sure he did too - that it was time for him to go in a different direction and we wish him well."