The first two days of the co-sanctioned event were dampened by lengthy storm delays but with the weather gods smiling down on the Sentosa Golf Club, the tournament reached a gripping climax over the final two rounds.
As an impressive Manassero threatened to run away with the event on a day when he ended up playing 36 holes, the 2010 Open Championship winner Oosthuizen emerged from the pack and could and should have won the tournament in the play-off.
The duo birdied the par-five 18th in regulation to finish locked on 13-under 271 and after they repeated the feat on the first extra hole, Oosthuizen had a four-foot birdie putt to claim the title on the next but his nervous effort horseshoed around the back of the cup.
Both men found the fairway on their third attempt at deciding a winner, with the South African's approach landing short and left of the flag before Manassero ripped a beauty that rolled down off a slope to within 10 feet of the hole.
Oosthuizen's eagle putt curled above the hole and his opponent made no mistake, picking up a third European Tour event before his 20th birthday with an assured putt into the middle of the cup that was met with a joyous roar from the packed gallery.
"It has been an extremely long day, the adrenaline kept me going and the play-off was just a great honour to play with Louis," Manassero said.
"It's the first play-off of my career and it has just been amazing really. Of course to end it with an eagle was great."
Almost forgotten in the mayhem was a brilliant final round six-under 65 from world number one Rory McIlroy that lifted the Briton up to third place on 10-under and secured the European Tour order-of-merit title with two events to spare.
"I played well all week but just didn't convert the birdie chances I gave myself," said the Northern Irishman, who was cheered on all week by tennis-playing girlfriend Caroline Wozniacki.
"I missed a lot of putts over the last four days. If I had holed a few more it would have been a different story. I am very happy how I played today and how I finished."
Halfway leader Thomas Bjorn of Denmark, who closed with a 68, finished a shot further back, and three-times Singapore Open champion Adam Scott of Australia was tied for fifth alongside Italy's Francesco Molinari on eight-under 276.
However, the day belonged to Manassero, who arrived at the course at the crack of dawn to complete his third round and immediately recorded four consecutive birdies on the way to forging a two-shot lead heading into the final 18 holes.
"Fortunately, the key was starting really well this morning after coming back on the course and I started with some birdies in a row which was very good and gave me a lot of adrenaline to go forward," Manassero added.
The 19-year-old Italian stretched his lead to three strokes by the turn as his closest challengers began to wilt in the scorching heat but Oosthuizen was made of sterner stuff and chipped away at Manassero's advantage until he took the lead with his birdie at the last.
Manassero, three groups back of Oosthuizen, had been in defensive mode since the fourth hole and after dropping a shot on the 15th, he needed to attack again and his short birdie putt on the 72nd hole set up the thrilling play-off.