Blues boss Alex McLeish has confirmed that he will have to name the Scotland international striker amongst his substitutes for the game at Oakwell on Tuesday evening because of injuries to other striker's.
Kevin Phiilips and James McFadden are still sidelined and Marcus Bent sustained a hamstring injury against Southampton at the weekend and is likely to be out of action for at least three weeks.
"I'm delighted to have him back," McLeish told the Birmingham Mail.
"It's been a long, long time for the big fella.
"He's still got to find some sharpness but I think he's getting through the period where there's a wee bit of tentativeness in his approach to things, kicking right through the ball.
"He's started to do that in training now and he's looking better with each touch he gets.
"He will be a big asset to us, another one who gives me a happy selection problem.
"I am hoping that in two or three weeks' time we are still sitting in a fantastic position and we have people like O'Connor, James McFadden and Kevin Phillips all chapping on the door again."