Celtic finished 21 points adrift of the title winners the season before O'Neill arrived.
Martin O'Neill during his time at Celtic
Martin O'Neill has told BBC Scotland (via BBC Sport) that Rangers' dominant title win during the 1999/2000 season helped him decide to take over as Celtic boss that summer.
The Gers had won the Scottish Premiership crown by a staggering 21 points during a season which had seen John Barnes sacked by the Hoops and Kenny Dalglish put in charge until the summer.
After enjoying a wonderful spell with Leicester City - the Foxes would gain promotion to the Premier League and win the League Cup twice during his tenure - O'Neill was appointed as the new Hoops boss.
And the boyhood Celtic fan confirmed that the chance to take the Bhoys back to the summit was an opportunity that was too irresistible to turn down.
"Naturally from where I was you were either Celtic or Rangers, brought up in Northern Ireland, and of course I was heavily into Celtic," he told BBC Scotland (via BBC Sport).
"So this opportunity to go and manage the club was something I didn't feel I could possibly turn down. Plus the fact they were in a bit of difficulty - I think Rangers had won the league by about 20 points and were really dominating Scotland.
"So here was this opportunity - a fantastic club, playing second fiddle to Rangers for some time and an opportunity to turn the tables."
O'Neill certainly restored the pride during his time at Parkhead. Celtic won three league titles, three Scottish Cups, one Scottish League Cup and reached the UEFA Cup final in 2003 during his tenure.
And his time with the club paved the way for Gordon Strachan to enjoy a similarly successful tenure immediately after him, so he will surely reflect that it was certainly a mission accomplished.