Donnacha O'Brien's Fancy Blue led home an Irish one-two-three in the Prix de Diane at Chantilly.
The former dual champion jockey only took out his training licence earlier this year, following in the footsteps of his father Aidan O'Brien - who completed the Derby and Oaks double at Epsom on Saturday - and his brother, Joseph.
Fancy Blue was a 13-2 shot to provide the fledgling handler with a first Classic success in the French Oaks, having filled the runner-up spot behind O'Brien senior's Peaceful in the Irish 1,000 Guineas three weeks ago.
The pair renewed rivalry in France - and were joined on the trip from Ireland by the Jessica Harrington-trained Alpine Star, who was the 6-4 favourite to follow up her triumph in the Coronation Stakes at Royal Ascot.
Drawn one, the latter made much of the running in the hands of Stephane Pasquier and did not go down without a fight, but Fancy Blue eventually wore her down under Pierre-Charles Boudot to claim a narrow victory.
Peaceful was just behind her compatriots in third.
O'Brien, 21, said: "There's so much pressure and a lot of work goes into it, but this feeling is great and thank God she won. I always knew she was a very good filly and we always thought the step up in trip would suit her. It definitely did.
"Pierre is a very good rider and I told him to do what he wants. There was no pace early so he went forward on her and sat upsides the leader. It was exactly what I would have liked him to do. She quickened up well and stuck at it. Fair play to Pierre, he gave her a peach of a ride."