The Northern Irishman played with Pat Jennings in his national side and won two European Cups and a league title with England's most capped player, Peter Shilton, at Nottingham Forest.
"I always compare keepers and Joe Hart has been absolutely fantastic lately," said O'Neill. "He is an exceptional talent and he is getting better as well.
"I like a brave keeper and he is one of those. Pat Jennings and Peter Shilton were just great keepers and I have had some very good ones over the years such as Tim Flowers, Kasey Keller and Brad Friedel."