Last year's Breeders' Cup Turf heroine Found was the marginal favourite at 2-1, bidding to get back on the winning trail after a hat-trick of runner-up finishes at Group One level against the boys.
Seventh Heaven was sent off at 100-30 in the hands of Colm O'Donoghue as she aimed to prove last month's surprise victory in the Irish Oaks was no fluke.
O'Brien fielded four runners in all and it was the least fancied of the quartet Pretty Perfect who took the field along for much of the mile-and-a-half journey.
As she began to falter halfway up the home straight, the Sir Michael Stoute-trained Queen's Trust moved smoothly towards the front, but the Ballydoyle big guns were waiting in the wings.
Seventh Heaven showed the best turn of foot to grab the lead and galloped all the way to the line to score by two and three-quarter lengths.
Found had to make do with second spot once again, with Queen's Trust a long way clear of the rest in third.
O'Donoghue said: "These are Ryan's (Moore) rides, hopefully we're looking after them for him and he's getting back soon.
"She's a very good filly. She was obviously a Classic winner coming here and was getting the weight allowance from some older fillies, so we made use of it.
"She's tough and genuine and has a great temperament and a beautiful action.
"This is what we all work every year for. It's nice to get the opportunity and to take it."
O'Brien was delighted with the performances of both the winner and the second.
He said: " We were delighted with Seventh Heaven at home and Colm gave her a beautiful ride.
"She came forward lovely from the Curragh and we thought the track and ground and trip would suit her well.
"The lads will decide what they want to do, but she's a lot of options.
"She's a beautiful traveller and is a massive, rangy filly that handles fast ground very well.
"She has plenty of class and has the option of all those fillies races and there's the option of the Filly & Mare Turf in America (Breeders' Cup)) at the end of the year.
"I'd say she's going to make a real four-year-old, because she's big and rangy."
Of Found, the trainer added: "She was just ready to start, but she'd an awful lot of class and we knew that. She got a little bit tired and Seamus (Heffernan) looked after her.
"The plan was to go to the Curragh (Royal Whip Stakes on Sunday) and then maybe go to Leopardstown (Irish Champion Stakes) and then maybe go to France for the Arc.
"We came out a bit earlier as we were going to leave US Army Ranger running by himself at the Curragh. Nothing might change from the plan.
"The lads will see the horses run at the weekend and the pattern should become apparent after that."
Chris Richardson, for owners Cheveley Park Stud, said of Queen's Trust: "It was no disgrace to finish third to those two fillies - a Classic winner and a Breeders' Cup winner.
"Michael said she seems to be effective over that trip as well as over 10 furlongs.
"We'll see how she comes out of the race and then see what to do. There are options abroad we may consider, but she will only get better."
Charlie Appleby blamed the ground for Endless Time's disappointing effort.
He said: "It was the ground. Her ability got her into the race, but she wouldn't let herself down on that ground."
Listed - East Cape Guineas
G3 - Kings Cup
G2 - Resource Recycling Superstars Championship
G3 - Lamb & Hayward 4&5yo Trotting Championship
G2 - Storer Motors Nz Trotting Oaks