By Geoffrey Riddle
According to the betting the 11-1 Magical was O'Brien's apparent third string but upstaged her two stable companions to take the Group Two.
Donnacha O'Brien dictated affairs on the sister of last year's winner Rhododendron and the daughter of dual Derby winner Galileo kept up the gallop throughout the seven-furlong trip.
Happily (6-4) tried to lay down a strong challenge but Magical would not be denied.
Willie McCreery's Mary Tudor was third, with the O'Brien-trained September, the 11-10 favourite, only fourth.
It was a 12th win in the race for the Ballydoyle handler, who said: "She had a lovely run the first day and she was a bit green at Cork and made hard work of it.
"Donnacha was delighted with her. She's out of a Pivotal mare and handled the ground.Magical is now the 12-1 favourite for the Investec Oaks with Bet365 (PA)
"Donnacha gave her a lovely ride and had her in a good position throughout the race. She stays well and should get well beyond a mile next year."
Of his beaten duo, he added: "Ryan (Moore) said Happily struggled to quicken and she is able to quicken better on better ground.
"Seamus (Heffernan) was a bit the same with September on her first run back. She will come on from that.
"They are all nice fillies and were all coming here with the view to coming back for the Moyglare.
"Clemmie will go for the Moyglare as well but not if the ground is soft."
Magical leapt to the head of the Invstec Oaks market and is a best-priced 12-1 with Bet365. With Happily eclipsed, Duchess Of Cambridge winner Clemmie is now the 8-1 favourite with 32Red for the Qipco 1000 Guineas. She is 7-1 with most other firms, while Magical is 12-1 with Bet365, Coral and Ladbrokes.
It was Moore who rode Found to lead home the famous one-two-three in the Prix de l'Arc de Triomphe last season and he cajoled Rostropovich at the death to win the Futurity Stakes.
Coat Of Arms had looked to have cut loose in the hands of Donnacha O'Brien, but Moore kept his mount going.
Rostropovich, a son of unbeaten Juddmonte superstar Frankel, was not the most willing ally but his jockey revved him up for one last push to score by a short head.
It was O'Brien's 11th success in the seven-furlong Group Two.
The Ballydoyle handler said: "He stayed at it well. He wasn't mad about the ground and will get further than seven (furlongs).Rostropovich is now a 33-1 chance for next season's Qipco 2000 Guineas (PA)
"On better ground he'll travel better. He was a bit laboured and the National Stakes (Curragh, September 10) is a possible."
O'Brien's domination was not total, however, and Dermot Weld and Ger Lyons hit the mark in the other two Group races.
Shamreen (11-4) comfortably disposed of the opposition to run away with the Group Three Qatar Airways Royal Whip Stakes.
Weld's four-year-old, blinkered for the first time, put the race to bed a furlong and a half out when asked for his effort by Pat Smullen after Massif Central had made the running.
The 33-1 outsider stuck on to claim second place.
Roger Varian's Central Square, the 6-4 favourite, was third of the five runners.
Weld said: "She won the Group Two Blandford last year and we may look at that race again for her.
"We will also be looking for a Group One, and the Prix de l'Opera and the EP Taylor in Canada are possible targets.
"The ground is vital as she is very effective on that ground.
"We found out last year at Cork in the Give Thanks that she doesn't stay a mile and a half, so 10 furlongs is her trip."
Treasuring stepped up on a nursery success on her latest start to lift the Group Three Qatar Racing And Equestrian Club Curragh Stakes at the County Kildare track.
Wearing the colours of Qatar Racing, the 3-1 shot was always close to the lead and had enough in hand to hold the 9-4 favourite Goodthingstaketime by half a length under Colin Keane.
Lyons said: "She has a great temperament.Colin Keane gave Treasuring a fine ride on Sunday (PA)
"If you are going to have a negative it's that she resents the stick. But she's always done that from day one, and she's trying her hardest for you anyway.
"She's a revelation and has done nothing but improve from her first run to here.
"She's out-and-out game and we've got winning black type for her, which is a great bonus.
"I can only imagine that she will keep developing into the winter and be a nice three-year-old.
"It's great for the owners ,as they own the sire (Havana Gold), and for the team in general."
Lyons and Keane earlier struck with Gobi Desert (3-1 favourite), who landed a gamble on his debut in the Irish Stallion Farms EBF Maiden.
The Oasis Dream colt was never far off the pace and knuckled down to see off Threeandfourpence and London Icon by half a length and the same.
Lyons said: "He's a horse that I'll bring back to six furlongs.
"This horse has a little bit of quality about him, and will be a nice horse for the future, if I still have him.
"He's a horse that we like and he likes the ground. He'll handle better. I'll concentrate on six until he tells me otherwise.
"He's a nice, straightforward horse and has plenty of stamina on the dam line."