But just as Swede Sebastian Soderberg looked poised to triumph after a stunning final day rise, Olesen sank a 30-foot eagle and a 36-foot birdie to snatch victory from the clutches of the collapse.
“Somehow I kept going. What a finish, unbelievable,” Olesen, who claimed €369,214 ($389,000) in prize money, told the DP World Tour.
Olesen had led from the front after a 6-under 66 on Thursday’s opening round, with Australian Ryan Fox joining him at the top.
A 2-under on Friday saw the 2018 Ryder Cup winner get a shot from German leader Hurly Long en route to day three, when a strong 3-under round gave Olesen a three-shot lead over Long and the English Marcus Armitage on his way to the last day.
However, a disastrous start to the final round saw Olesen open with a bogey, with a birdie on third providing a breather for another four bogeys before the 16th hole.
“It was a pretty rough day, a lot of shots to the left – I had a really hard time,” Olesen said.
Soderberg denied despite beautiful Sunday
Meanwhile, Soderberg enjoyed a great lap that lifted him up the standings and into battle. After getting six shots behind Olesen on Sunday, the Swede scored the best 68 on the final round to lower the clubhouse goal at nine.
After improving par only once to the first 16, Olesen now needed two straight birdies to force a playoff.
Given that the Dane had carded an eagle birdie to close out on Saturday, a replay might not have seemed so miraculous, but after a bogey on two of the previous three holes, it seemed impossible.
And yet Olesen repeated the feat to take a dramatic win, turning around to celebrate with a packed crowd at the finish.
The win moves him up 69 places to 9th in the DP World Tour rankings and joins golf icons Seve Ballesteros and Nick Faldo in winning the British Masters.
“Obviously it’s a huge tournament, it always has been,” Olesen said. “There are so many great champions here, so it’s a privilege to get your name out there.
“It’s always great support you get in Britain. The crowd has been amazing from day one and then… so much support, so I’m really happy about that, it means a lot.”