In the Stadio delle Alpi in September 2001, in their first outing in the group stages of the Champions League, Celtic recovered from a 2-0 deficit to peg back Juventus at 2-2, before Amoruso took a tumble in the final minute of the game. He then got up to dispatch the penalty.
O'Neill was incandescent in a post-match television interview and, while time has dulled the fury, it has never taken away his sense of indignation.
"I do admit it was definitely not a penalty," Amoruso said this weekend. "I had only been on the pitch for a few minutes, there was one ball in the box, and the Celtic player was near me. It was never a penalty, but the referee gave me the decision and I scored."
It was a decision that cost Celtic dearly. The club did not qualify for the latter stages of the competition that season, despite finishing their maiden campaign with an impressive nine points. And, even after all these years, it irks O'Neill.
"Absolutely. It still rankles, no question about that," said the Irishman. "It really does."
Juventus have fought back to the top of Italian football after being relegated in 2006 for the first time in their history due to their involvement in the Calciopoli match-rigging scandal which rocked Italian football.
Juventus were disgraced, humiliated and banished to Serie B for their role and it is only now that they look as though they have fully recovered their former poise. The penalty decision, however, still troubles O'Neill.
"You know, as the years go by there is some concern about the result in your mind," he said.
Neil Lennon, too, has never forgiven or forgotten. At a Hampden lecture last season by former Juventus manager Marcello Lippi, who was in charge of the Italians in 2001, Lennon was quick to remind him of Amoruso's dive.
It certainly draws an emotional response from O'Neill, certain his team were cheated out of the game. "When you look at that game in Turin, we got it back to 2-2, we did so well to do that, and then they got a last-minute penalty that even to this day I still believe not a soul in the ground looked for to be given," he said. "I remember turning away, waiting for the goalkick to be taken, and just being stunned. Absolutely stunned. There is no question that even after all these years that is still there – that sense of injustice about it."
"That game in Turin was our first ever Champions League game," O'Neill added. "Our original first match, which was due to be played at Celtic Park against Rosenborg on September was postponed because of the 9/11 tragedy in America. It really left a sour taste and I do have cause to wonder about it."