Gardner, 25, has taken time to settle into life at the Stadium of Light following his £5 million summer switch from Birmingham.
However, his performances in the last four games, several of which have seen him deputise at right-back, have seen him return to the kind of form in which then manager Steve Bruce invested so heavily.
Successor O'Neill has seen him emerge from the shadows to establish himself in the side, and his performances have not gone unnoticed by the club's fans.
"I want Craig to stay at this football club.'' O'Neill told the Sunderland Echo. "I don't think he is unhappy, but I do think he has felt a little homesick.
"But the crowd have given him a great reception in the last two games, and that was great for him. It will have given him a big boost. And, like most players, if you are playing games, then it helps enormously.
"Perhaps it didn't happen for him here straight away and maybe homesickness had something to do with it, but he is starting to play well now.''
O'Neill's stance may prove disappointing for the likes of Wolves and West Brom, who have both been linked with Gardner in recent weeks amid speculation that he could be seeking a return to the midlands.