While Australia signed off on their preparation with confidence-boosting victories over South Africa and the All Blacks, the Kiwis enter rugby's showpiece event with rare back-to-back losses.
"It was very significant," O'Neill said of Saturday's 25-20 triumph over the All Blacks in Brisbane.
"It was a great Test match, and clearly a great result. The timing was perfect. It gave us momentum going into the World Cup in 10 or 11 days time.
"When it was 20-all with 20 minutes to go, they're the sort of games we may have lost in the past, but we won and there was a lot of character and composure in that result.
"A lot of us have been keeping the faith and believe the win in Durban, backed up by the win at Suncorp is a sign that the faith that we've placed in the coach and the players is starting to come good.
"Tournament rugby... is in some ways very different but to win a Test match like that against the best in the world, it had all the tournament-style rugby aspects attached to it, and our blokes didn't flinch.
"Is it a key indicator? Well look, I'd rather have that momentum than not have it."
O'Neill famously claimed that all Australian sports fans "hate England" in the lead-up to the Wallabies' 2007 quarter-final loss to the old enemy, with some England players conceding they used his words as motivation ahead of their upset victory.
The veteran administrator was understandably cautious not to give the Kiwis too much ammunition ahead of a possible World Cup final showdown on October 23 when asked if Saturday's victory sent a shudder through the New Zealand camp.
But he was quick to remind Graham Henry and co of their recent back-to-back losses to South Africa and Australia.
"Everyone will say what they need to say but we just have to keep our minds on a game by game approach, really looking forward to Italy on Sunday week and then onwards and upwards," he said.
"Ireland in that second game at Eden Park will be a real contest. I was lucky enough to be there to see Ireland beat England in March, and they're a very formidable side.
"So there's no easy games... but the All Blacks have lost two in a row, which is unusual in the very high standards that they set themselves."
Asked if anything less than a third World Cup triumph will be deemed a failure, O'Neill said without hesitation: "That's why you go into these competitions. The bookmakers price these things pretty accurately and the All Blacks are still favourites - we're second favourites.
"I don't mind being the second favourite - it's not a bad position to be in - but throw in England, France, South Africa, and throw in Samoa - we were only beaten by them a few of weeks ago.
"I think it'll be a great tournament.
"We're in with a real shot but let's not get ahead of ourselves."
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