Having broken well under Ryan Moore, registering his first success in the £1-million race, the well-backed 11/8 favourite always travelled kindly on the rail in behind market rival Galileo Gold. Turning for home, the son of Galileo looked to be travelling best of the 10 runners but got slightly caught for room approaching the two-furlong pole.
However, once Moore found daylight on the French Guineas winner, The Gurkha responded well to his jockey's urgings to quicken up nicely and battle on gamely all the way to the line, fending off the late challenge of Galileo Gold by a neck with Godolphin's Ribchester a further short-head back in third.
O'Brien, registering his fifth success in the Group One feature and his 15th victory at the Qatar Goodwood Festival, said: "Ryan was prepared to make the running today if no-one was going to make it but I presume he was happy where he was."
The Ballydoyle handler was quick to point out that the three-year-old's run last time on soft ground in the 10-furlong Coral-Eclipse was a far from ideal preparation. He commented: "We always thought he was a very fast horse, he has an awful lot of speed. He was always a fast Galileo. But we took a chance at Sandown over a stiff mile and a quarter in soft ground and we were just worried that it took a lot out of him but from day one he has always been very quick.
"He is a good traveller who handles fast ground very well and quickens very well. I suppose he is also very courageous which is why he has run the races he did like when we took him out of his comfort zone at Sandown [Coral-Eclipse] and even at Royal Ascot where the ground was soft.
"Obviously, we know he goes in soft ground but he is at his best on fast ground. I think he'll probably go back and get a mile and a quarter but his ideal trip is a mile I imagine. In an ideal world, we want to keep him on fast ground. He is a very low fast-moving horse.
"It has been tough on him. The lads have been happy with him. But I was worried about him as he has had such a big career crammed into a very short time. He has danced every danced all the way along and he has been turned out plenty. I'm delighted. Ryan gave him a brilliant ride.
"We have been hard on the horse. We haven't wrapped him up or protected him. The soft ground at Sandown got into him a little bit."
As to the future, O'Brien outlined a list of potential targets for the exciting colt. He said: "The lads will decide, if he needs to go back up in trip, he won't mind going back up particularly if it is fast ground as you wouldn't mind going back up at a fairly level track. He has lots of options.
"He could go to France for the mile race, Leopardstown for the Champion Stakes or head to York [Juddmonte]. Obviously, York would be the easiest mile and a quarter and Leopardstown would be the stiffest mile and a quarter. I suppose we will have to see how he comes out of today."
O'Brien added: "The horses are running really well and they are coming on from run-to-run."