"It is absolutely extraordinary," O'Neill, who led the Foxes to two League Cup triumphs, told ITV News. "The team has lost three games. That in itself is a fantastic achievement. But to go and win the Premier League in this day and age when money seems to count, it is absolutely fabulous.
"Romanticism is still alive in football and that is great news for everyone.
"Leicester City have shown the way, shown how it can be done and great credit to all of them. To the players, the manager who has been terrific, the back room staff who have assembled the team. It's been a team effort from start to finish."
Claudio Ranieri's side will receive the Premier League trophy this weekend after their home match against Everton.