The 22-year-old has had a successful spell at Bramall Lane and has caught the eye of other clubs.
"Gary's situation was that he could spend 30 days or 90 days on loan at Sheffield United and he has come back here for the Christmas programme," O'Neill told the club's website.
"But I will sit down with him and talk with him.
"There is more than a measured interest in him because he's done very well at Sheffield United and that was always going to be the case anyway, but we will make that decision.
"There's quite a substantial amount of interest in him and I think Gary is aware of that because people have been speaking to his agent, so I will certainly be sitting down with him in the lead-up to the games.
"I will sit down and speak to him to run through things with him. But it's only fair I speak to him first."