Northern Ireland crashed to a humiliating 3-2 defeat to Luxembourg only a month after beating Russia. It was the Duchy's first victory in World Cup qualifiers in 41 years.
However, the home side enjoyed the best of the first half, hitting the bar through Lars Gerson and deservedly equalised in the 45th minute when Aurelien Joachim's shot deflected in off Daniel Lafferty.
Luxembourg pressed again after the break and just when it seemed as though their momentum was fading, Stefano Bensi put them 2-1 ahead with a stunning 25-yard drive.
Gareth McAuley looked to have spared Northern Ireland's blushes with an equaliser with eight minutes to go, heading in Oliver Norwood's free-kick at the far post, but Mathias Janisch clinched a historic victory when he scored from close-range in the 87th minute.
O'Neill was controlled but simmering beneath the surface as he offered a scathing verdict on what he had seen and the positivity of last month's morale-boosting 1-0 win over Russia seemed an age ago.
Afterwards, he said: "It was very poor, we were disappointing from start to finish. The performance was unacceptable, the energy and work rate was not at the level required.
"It was a really poor performance - we lacked energy, intensity, intelligence. The management of the game from start to finish was pathetic," he said.
"We got what we deserved. We didn't deserve anything...Luxembourg were the better side. It was a really, really poor performance.
"There was not a single positive out of that game."
Asked to put his finger on a precise reason for the way his side were outplayed by a side languishing 140th in the FIFA rankings, O'Neill had plenty.
"Lack of energy, lack of desire to play, lack of intensity, lack of leadership. Take your pick," he said.
Northern Ireland were cheered on by an impressive away contingent of 1,200 - a group who did not turn on the side even as the game slipped from their grasp in the second half - and O'Neill offered a frank apology to them.
"I feel for the people who have travelled to watch that," he said.
"It's not acceptable and not the level of performance I would expect from my team.
"The players are pretty clear on my feelings. As a manager and a group of players we owe the fans an apology."
"When we drew the home games against Luxembourg and Azerbaijan I had a certain level of sympathy because I felt on the night we did a lot of positive things, but this was as poor a performance as I've ever been associated with," he said.
"If we are at it all of the basic ingredients for playing football are there and we've shown what capable of. But if we're not we have no right to think we're better than Luxembourg or Azerbaijan or anyone out there. That has been proven."
Luxembourg boss Luc Holtz was effusive in his praise for the hosts, who last won a World Cup qualifier in 1972.
"This was one of the best games our team has seen in a long time," he said.
"Northern Ireland's first shot on goal went in, so we were unlucky, but in our eyes our team played much, much better.
"It was a very good performance. The whole team was brilliant.
"We like to play the attacking way and the players like to do this too. This is the way we will play in the future."