Piggy French won the London 2012 equestrian venue's test event in Greenwich Park with two clear show jumping rounds aboard DHI Topper W.
French, 30, from Norfolk, led throughout the three-day event, beginning with Monday's dressage then a faultless 3km cross-country phase.
Fellow Briton Pippa Funnell came third, behind Germany's Michael Jung.
"It's amazing to think that, this time next year, we could be riding for our country in this setting," said French.
Greenwich Park has been well-received by riders and protests against its use as a 2012 venue have been minimal.
The park will now host modern pentathlon's test event, the 2011 World Cup Final, on Saturday and Sunday.
The pentathlon test event's equestrian and run-shoot disciplines will take place in the park, with the opening fencing and swimming rounds held at the Crystal Palace national sports centre. In 2012, the fencing and swimming will take place inside the Olympic Park.
Once the pentathlon test event is complete, the park's equestrian features will be dismantled until early in the summer of 2012.
Some 3,000 schoolchildren were handed free tickets to this week's equestrian test event, in the form of a three-day event featuring dressage, cross-country and show jumping.
While the Olympic tournament will receive a four-star rating - the most difficult in three-day eventing - the test event is an easier two-star competition, making use of a shortened cross-country course and offering riders the opportunity to try the venue with younger, developing horses.
If selected for the Games, for example, French is expected to prefer top horse Jakarta over the likes of DHI Topper W.
But French, the runner-up at this year's prestigious Badminton horse trials told BBC Sport the horse still has a chance.
"DHI Topper W could easily be a 2012 horse," she said.
"He's a little bit below the level of my top horses at the minute but improving all the time.
"It's excellent for him to come here, do this and get the experience."
The battle now continues for selection to the British eventing team for the Olympics, with more than a dozen riders still in the hunt for just five berths in the squad.
Four of the top five eventers in the world, according to the rankings at the time of writing, are British: Mary King (first), William Fox-Pitt (second), French (fourth) and Oliver Townend (fifth). Funnell is ranked 11th.
The British team for the European Championships in Luhmuhlen will be announced early next week, following the weekend's Barbury Castle horse trials.
"In eventing we've got a real challenge to get into the team," said Fox-Pitt, who finished 10th in Greenwich.
"It's about producing the horses, getting the results and keeping the horse and rider in one piece.
"It's been very exciting being here. I was a little bit nervous today, but nothing compared to what it will be like in a year's time."