O'Brien enjoyed his 900th Group/Graded winner under both codes as Ryan Moore led Broome to a comfortable victory.
Last season's Irish Derby and St Leger winner Hurricane Lane might have been considered as the big favourite for the mile-and-a-half Group Two feature, however he showed little condition for his first run of the season on ground quicker than ideal.
Moore's aggressive strategy from a wide draw proved faultless as he took the sting out of the Charlie Appleby runner.
Broome, who landed last year's Group One Grand Prix de Saint-Cloud in July, kept plugging away and as Hurricane Lane slowed, the 6/1 chance took full advantage.
Mostahdaf and Jim Crowley gave the eventual winner a late challenge, but they ultimately came up three and a quarter lengths short.
After the race, O'Brien - who moved to 81 career winners at the Royal meeting - said: "I didn't realise that (900 top-level winners) and we are so privileged to be in the position we are and we are so grateful to everyone at Ballydoyle.
"In this life and world we take one day at a time and try to appreciate every day. We're so grateful to do what we do every day. It's incredible really."
Paying tribute to Moore, he said: "It was a brilliant ride. Ryan has done an incredible job on Broome. He is a six-year-old colt now and one very strange thing is that he seems to be coming to the peak of his career after all these years. He is by Australia and is very brave.
"We think we will see the best of him this year. Ryan gave Broome an amazing ride."
O'Brien indicated the King George VI And Queen Elizabeth Stakes will be the target for Broome, and added: "He suffered a fractured shin when he was kicked unsaddling after the Japan Cup.
"He had time off and needed his run in the Tattersalls Gold Cup.
"He has a very high cruising speed, and doesn't surrender."