Claudio Ranieri's men are currently three points clear at the top of the Premier League table.
Claudio Ranieri's side were tipped by many at the start of the season to be one of the favourites to be relegated this term, given their struggles last year, as well as the upheaval that was seemingly caused when influential manager Nigel Pearson departed the King Power Stadium.
However, with 10 games to go, Leicester are currently three points clear of Tottenham, while Arsenal are six points back and both Manchester clubs are 10 points adrift of the surprise pace-setters.
And O'Neill - who managed the club during arguably their most successful period in the late 1990s, as they won the League Cup on two occasions - believes that if they can finish the job they have started, it will be one of the greatest stories in football.
"It's the story of the decade without a doubt, and it's been absolutely fantastic. If everything went poorly from here on in - which it won't do - it still will have been a magnificent effort," he told Sky Sports.
"At this stage they're almost - almost, it's not there yet - guaranteed Champions League football. It's been absolutely fantastic."
Considering at this stage last year the club were rooted to the bottom of the table, and well adrift of safety, O'Neill is surely right to suggest that them winning the title would be one of the most sensational results in recent history.
And based on their remaining games, they arguably have a wonderful chance of holding off the challenge of the chasing pack. They face none of the other teams that are currently in the top four, so may feel much more confident of picking up the points they need to get across the line.