McLeish is hoping striker O'Connor's second loan spell with Barnsley will increase his match sharpness.
He said: "We just feel he is a bit rusty. He is not going to get regular first team football here at the moment.
"Garry is looking for regular football and he asked to go back out on loan and I said fine.
"To be fair to the big man, he came in for the Blackpool game and did well.
"I was hoping he would move forward but, after the Brentford game in the Carling Cup, he said he needed more time to get match fit."