O'Neill turned to the 25-year-old as defensive injuries, and particularly the ankle problem which has sidelined first-choice right-back Phil Bardsley since pre-season, bit hard into his resources, and his response has been impressive.
Gardner had previously taken on that role on occasions during his time under the Ulsterman at Aston Villa, but his manager believes he is better equipped this time around.
O'Neill said: "He has done very, very well indeed, Craig. He is essentially a midfield player. I probably didn't expect Phil Bardsley to be out for the length of time that he has been - he injured himself in the first three days of pre-season training.
"Craig has filled in there and he has done brilliantly for us, really brilliantly, and I couldn't really ask for any more.
"He has learned that position. He did fill in on occasions at Aston Villa there, but he is a stronger player now and I think he is a better player now. He is less daunted by it too. He always felt that wingers were coming at him left, right and centre. Now he is capable of coping with it."