O'Neill, who also worked with 25-year-old Gardner during his spell as Villa boss, insists he is keen to prove himself on Wearside.
"I know Craig from our days together at Aston Villa. He really is a great character," O'Neill told Sky Sports News.
"He's been a family man for a long, long time. His family are down in the Midlands.
"I think he wants to do his best to settle here and prove the move successful."