"We are disappointed he didn't finish closer but there is still plenty of horse to go on with."
Yesterday, So You Think was never in winning contention after having to settle near last from his wide barrier.
Now trained in Ireland by O'Brien, the son of High Chapparal ran home strongly but was no match for runaway winner Danedream, with the 3-year-old filly, by Lomitas (GB) from Danedrop (IRE), winning by five lengths in race-record time.
Outsider Shareta - whose stablemate and favourite Sarafina never threatened - was second, with four-year-old mare Snow Fairy a fast-finishing third.
So You Think's stablemates, Treasure Beach and St Nicholas Abbey, tried to give the former Bart Cummings-trained galloper every chance by setting a solid pace.
O'Brien was not willing to give an immediate appraisal of the eight-time group one winner's performance immediately afterwards. "I will have to look at the replay of the race but if the owners are willing I would like to set him towards the Breeders Cup."
So You Think's jockey, Seamus Heffernan, said it was a tough race to judge.
"I wasn't in a great position coming into the straight, but if there was one thing I might wish to do over again it would be to have got into a better spot in the early part of the race," he said.
Danedream's trainer Peter Schiergen said the filly was the best horse he had trained.
"She is a small filly with a big, big heart and there's no better race to win. This is a really big day for German racing."
Of the other fancied runners, seven-time winning Arc trainer Andre Fabre's Meandre never looked likely to add to that tally as he was pulling from the off and finished seventh.
Highly fancied and well-backed Japanese runner Hiruno D'Amour was prominent but stumbled coming into the straight and was then baulked leaving him with no chance and leaving Japan still without a win in the race they dream about winning.
Also on the card, French racegoers were denied the fond farewell they were hoping for in the Prix de la Foret as the great Goldikova was touched off by Dream Ahead, who landed the fifth group one of his career after a thrilling finish.
The son of Diktat will not race next season and may have run his last race.
Owner Khalifa Dasmal has sold part of him to American investment group SF Bloodstock and Ballylinch Stud, whose manager John O'Connor said: "He won't race next season, but he does have options this year like the Breeders' Cup and Hong Kong so we'll see."
Goldikova had the run of the race, slipping neatly up the rail in the straight as her pacemaker Flash Dance gave way.
Italian challenger Worthadd also dropped back under Frankie Dettori, but July Cup winner Dream Ahead was travelling extremely well before being pulled out by William Buick, who had already struck in Group 1 company when Elusive Kate took the Prix Marcel Boussac.
The giant-striding Dream Ahead powered alongside Goldikova and looked likely to win decisively, but it was to be a ding-dong struggle as not only did the David Simcock-trained 3-year-old hang a bit over the track, but his formidable rival fought back in determined fashion.
After a fierce final-furlong battle, they passed the line in unison, only for a photo to reveal that 7-2 shot Dream Ahead had nailed the odds-on favourite.
Given the exploits of Frankel, it is easy to overlook that Dream Ahead was joint-champion 2-year-old last term after winning both the Prix Morny and the Middle Park.
Longchamp's seven-furlong track has sometimes not seen Goldikova in her best light and she was beaten in the Foret two seasons ago before making amends in 2010.
On both occasions she went on to win the Breeders' Cup Mile, and it is doubtful yesterday's defeat puts any dent in her claims as she bids for a wonderful four-timer in the States at Churchill Downs next month.
"Maybe she does not have the same turn of foot that she used to and maybe the mileage is starting to catch up. If she recovers well we'll probably go to the Breeders' Cup," said her trainer Freddy Head.
-AAP
