The 1-2 favourite, who took the spoils by half a length to follow up subsequent St Leger hero Kew Gardens' victory in the same ParisLongchamp Group One last year, was also completing an outstanding weekend for Ballydoyle - after Ten Sovereigns' July Cup win at Newmarket on Saturday.
Japan, winner of the King Edward VII Stakes at Royal Ascot last month, bided his time in third under Ryan Moore behind pace-making stablemate Western Australia before going to the front more than a furlong out and holding on from home hope Slalom.
O'Brien reflected on a race which appeared to go very much according to plan for the Galileo colt, who was striking at the top level for the first time following his two previous Group Two wins.
He said: "We were very happy. It wasn't a strong pace, but Ryan was prepared for that - so he got in a handy position.
"He's a horse who never wins by very far - but at Ascot he won by a bit further because the pace was very strong, the field came back to him and he was left in front."
Japan may well be back at ParisLongchamp to challenge for the Prix de l'Arc de Triomphe in October - and O'Brien is already mapping out his interim route, which will first include discussions with his Coolmore owners.
He added: "The lads will decide what's next, but we were thinking of a little break.
"We rushed him through the whole year to get to here - to the Dante, the Derby and then Ascot - so we thought about the break after this.
"We'll maybe look at the (Irish) Champion Stakes or an Arc trial around that time, something like that."
Japan, third in the Investec Derby at Epsom before his Royal Ascot victory, is 9-1 from 12-1 for the Arc with Paddy Power following Sunday's win.
Winning jockey Ryan Moore told Sky Sports Racing: "He's a really beautiful horse - everything he does is lovely.
"He's a beautiful mover, a well-balanced horse - and he was always doing what he had to do today.
"I didn't have to get too serious with him, just gave him a couple of little flicks to keep his mind on the job."
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