The man who brought Lennon to the club in 2001 believes his achievement is taking the side he has built to the Last 16 of the Champions League confirmed his credentials.
And O'Neill reckons the Hoops were unlucky not to go even further.
In an interview for a new book, Faithful Through And Through, O'Neill covers a wide range of subjects, including his five years in Paradise, the run to Seville, and why the club could not answer the fans' calls to push on from a position of power.
But, it is when speaking about how Lennon now manages Celtic that O'Neill hands out the plaudits.
He said: "The club is in good hands now with Neil as manager.
"He has overcome some uncomfortable moments, as you might expect from someone embarking on a managerial career.
"And he continues to lead the club as a manager in the same way as he did as a player - from the front.
"He knew the club and the history, and we disagreed vehemently a few times. But, over the period, I thought we had a really strong relationship.
"I'm genuinely delighted it's going well for him as manager of Celtic. He could become one of the all-time greats."
As the only Celtic manager other than Jock Stein to lift the Treble, O'Neill is already considered among that group.
But, the trophy haul could have been even greater, had the club not, effectively, hit a financial wall after Seville.
That still annoys many supporters, but O'Neill explained: "I understand what a lot of fans were saying, that even though we got there we would continue to be pressing on.
"But I think a lot of things had changed in that time. There wasn't any money - Celtic and Rangers could have gone their own way in terms of TV revenue with league games, and that might have earned them more.
"I know that suddenly there didn't seem to be any money in the SPL between 2003 and 2005.
"We tried with the Wolves player, (Henri) Camara, and we brought in Juninho, but he was at the end of his career.
"In essence, neither Celtic nor Rangers were spending. The days of the big six-million-pound signings, like Chris Sutton and John Hartson, had gone, and it was all done with TV revenue."
Under Lennon, and without the pressure placed on the Hoops by Rangers, O'Neill believes they can go from strength to strength.
"Amazingly, Celtic's results in Europe were incongruously brilliant last season," said the 61-year-old.
"Neil might argue he was able to concentrate more because not every result was absolutely vital in the league.
"In my day, the week before a European match, we might have felt absolutely under pressure to get league points.We needed 90-odd points, and even then we were not certain of winning.
"Last season, they won it with 79 points. However, they had some tough European qualifying games, and these are difficult matches to be playing early in the season.
"To then go into the Group Stage of the Champions League and win in the way they did against Spartak Moscow in Russia and Barcelona at home was really something fantastic."
Faithful Through And Through, by Richard Purden, published by Headline.