The 26-year-old scored 18 goals for the Bees in League Two this season, his first in English football following a successful stint with Cork City.
But veteran goalkeeper Harrison, who also serves as an assistant to manager Ian Hendon, is sure O'Flynn will start the new campaign with Barnet, but accepts he may not finish it with them.
"I was at Peterborough when he was there as a youngster and he had the potential then," said Harrison. "I spoke to Barry Fry recently and he said that because John was always injured at the time he didn't want to take a risk on him.
"He went to Ireland and it was our commercial manager who works for Cork that told us about him. He said they were in financial trouble and that he might be worth a punt. We did and he's turned out to be one of the best and most intelligent strikers in our league. He knows where the goal is for sure.
"We'll be fortunate enough to keep him because it was in the second half of the season he did it. But if he starts well, it could be January when we definitely get some bids for him I think."