A closing 72 for an eight-under-par 276 total saw Casey become the first player to record back-to-back wins in the event since it began in 2000.
He finished one shot ahead of 2010 Open champion Oosthuizen (69) and American Jason Kokrak (71), with Johnson ending up three shots back following a 74 – his highest closing round in 18 months and one without a single birdie.
Casey started the day with a one-shot lead over Johnson, who was bidding for his second victory in three starts after winning the WGC-Mexico Championship earlier in the month. As the Copperhead Course in Innisbrook Resort in Palm Harbor continued to prove a stern test in the final leg of the PGA Tour’s Florida Swing, Oosthuizen threw down an early gauntlet by holing out from a bunker for an eagle-3 at the opening hole.
Casey made birdie there but had slipped to one-over for the day following back-to-back bogeys at the sixth and seventh and was joined at the top of the leaderboard by Oosthuizen when the South African birdied the par-5 11th.
An up and down from a bunker saw Casey match that to go ahead again, but, after finding sand again at the par-3 13th, a bogey there dropped him back into a share of the lead with Oosthuizen and Kokrak after the American birdied both the 11th and 13th to move to eight-under.
While it was impressive to see 41-year-old Casey matching Johnson, his playing partner, for length off the tee, his short game was the key to this win. A brilliant chip from the side of a grassy bank set up a birdie at the par-5 14th to edge him ahead again and his touch around the green saved par from a nasty spot two holes later. He three-putted the par-3 17th to make it a sweaty finish but it was job done after closing with a par following a brilliant fairway bunker shot.
After being two-under for the day early on, Scottish No 1 Russell Knox dropped four shots in the final 11 holes as he shot a 73, dropping him 11 spots into a tie for 24th on one-under.
Elsewhere, Australian Scott Hend secured his third win on the European Tour after beating Spaniard Nacho Elvira in a play-off following a dramatic finish to the Maybank Championship in Malaysia.
As the heavens opened over Suajana Golf and Country Club in Kuala Lumpur, overnight leader Elvira was playing his approach at the last as the first clap of thunder was heard, leading play to be suspended for an hour and 40 minutes.
When it resumed, Elvira coolly rolled in a 30-foot birdie putt to join Hend on 15-under-par but it was Hend who prevailed with a birdie-4, helped by a slice of luck when his drive hit the base of a tree and rebounded back into the fairway, when the pair returned to the 18th in the sudden-death shoot-out.
“I’m a grinder and a fighter. So is my caddie Tony,” said Hend, who carded back-to-back 67s in the final two rounds. We both have the same mental attitude. It doesn’t matter how old we are, we work hard at what we do. Just takes a bit of luck to win. Obviously I had a bit of luck on the play-off hole. If you don’t have any luck you won’t win.”
It was Hend’s first victory on the circuit since landing the True Thailand Classic in 2016, two years after his breakthrough success in the Hong Kong Open. “Can’t ask for much more on a Sunday when the golf course is playing quite tricky,” he added, having began the final round three shots behind Elvira before racing to the turn in 31.
Elvira’s runner-up finish was his second in as many starts after also getting himself in the mix in Qatar in his last outing. “It was thunder in a bad moment, but the gods did give me one back with that putt,” he said.
David Law’s closing 74 left him in 72nd spot on four-over-par.