Following placed efforts in the Derby at Epsom and the Irish equivalent at the Curragh, Cracksman was seriously impressive when winning the Great Voltigeur at York and the Prix Niel at Chantilly.
Connections sidestepped a clash with Enable in the Prix de l'Arc de Triomphe, which looked a wise call after the brilliant filly produced an imperious display, and the decision was further vindicated when Cracksman dominated his rivals in the Qipco Champion Stakes.
Oppenheimer said: "I think we were all a little bit stunned by his performance on Saturday, to be honest. To see him win by the distance he did was a surprise.
"It's hugely exciting. We know he stays a mile and a half well and coming back to the mile and a quarter was a little bit of a worry, although Robert Havlin had said he felt he was getting quicker.
"He's come out of the race incredibly well. It's a bit frightening, actually.
"I think he'll just wind down gently now and we'll look forward to next year."
Cracksman will be given a well-earned break ahead of his 2018 campaign, which is likely to include two major races.
"We're very keen on the King George, obviously, as we used to sponsor the race, and the Arc is another big target. Apart from that we haven't really discussed what we're going to do," Oppenheimer added.
"There are certainly more options for him now we know he's effective over a mile and a quarter. We have a wider sphere of races to consider.
"He's a very strong horse now and he's coming into himself. We always felt he'd be a better horse next year, although he looks fully furnished now."
Cracksman's stellar campaign comes just two years after Golden Horn carried the same colours to Derby and Arc glory and Oppenheimer was keen to pay tribute to both trainer John Gosden and jockey Frankie Dettori for their roles in his success.
He said: "To have two horses like Golden Horn and Cracksman so close together is extraordinary really. We are so lucky.
"Both myself and my father have had some very good horses over the years, but I think the Dettori-Gosden factor has added an extra dimension.
"With Frankie and John and everyone at the stud, we have a top quality team and obviously luck plays a huge part as well."
On the potentially mouthwatering clash with Enable in 2018, Oppenheimer said: "If we hadn't won the Arc a couple of years ago with Golden Horn, maybe Cracksman would have taken on Enable at Chantilly, but we felt that could wait for another day.
"We knew Enable had worked seriously well before the Arc and we felt missing the race this year was the right decision for our horse.
"L et's hope both horses stay safe and sound and they can maybe meet in the King George or the Arc next year on genuine good ground. That would be something to look forward to."