Thomas was able to stave off advances from most of the pack, including Patrick Reed who was looking over Thomas' shoulder until the very end. But he'll have to try for it again next year.
We say "loser" in the most positive way possible for Oosthuizen, who shot 70 to finish T-2. The top contender among them is American Chris Stroud on five-under overall with five front-nine holes remaining.
He can count on two FedEx Cup playoff events, but he'll probably have to show some form to advance through the PGA Tour's postseason and, more importantly to him, give USA captain Steve Stricker a reason to pick him for the Presidents Cup.
It was Thomas' third title of the year, following his back-to-back victories at the SBS Tournament of Champions and the Sony Open in January. At the time, McIlroy was 25, ranked No. 1 in the world and the third-youngest player - behind only Tiger Woods and Jack Nicklaus - to have won four majors.
The victor might have to courageous the daunting three-hole stretch a few times. Kisner, the 54-hole leader, closed with a 74.
It easily could have been a huge negative momentum swing for Fowler, who had birdied the previous two holes.
Kevin Kisner held the lead for the first three days of the tournament but wilted under the pressure on Sunday and finished at 4 under.
"The putt on 10 obviously snuck up on the hole", Thomas said. He played them in a combined 6 over par on Saturday and Sunday, including a double-bogey 6 at No. 18. That was a long way from securing a career Grand Slam in his first attempt after he won last month's Open Championship at Royal Birkdale.
So much for Quail Hollow's setup causing a yawnfest over the weekend.
Even the most casual fans could notice the host of big-name golfers waiting to congratulate Thomas on the 18th green.
The 24-year-old - whose father is a club professional and whose late grandfather was a long-time professional who qualified for the 1962 US Open - became the eighth first-time major victor since 2015.
"The only putt I made was on the last hole", Oosthuizen said. McIlroy, Matsuyama, Spieth, and others were all being lined up as favorites to take the win.
The American began the day sharing the lead with Japan's Hideki Matsuyama at the Quail Hollow Club in Charlotte, North Carolina. Matsuyama is the world No 3 and one of the best golfers in the world without a major title while Stroud has only previously made the cut twice.
But you have to hand it to him. After opening up with a score of 73 on Thursday, Thomas zoned in.
Social media fired up over whether Thomas was within the rules to claim the birdie, given under the rules a putt has 10 seconds to drop.