The top-three finishers form the event will get a chance to participate in the Commercial Bank Qatar Masters Pro-Am tournament.
Grace began the back nine of the final round at Randpark Golf Club at 17 under par and level with fellow South African and defending champion Louis Oosthuizen.
Ormsby surged ahead in early play, carding a four-under 66 to go nine under and stay top of the leaderboard at the historic Fanling course where he won his maiden European Tour event in 2017.
The Northern Irishman and reigning PGA Tour Player of the Year returns to the oceanside course outside San Diego after finishing fifth in the tournament in 2019.
The 26-year-old American edged Patrick Reed and Xander Schauffele in playoff for his 12th US PGA Tour title.
Qatar's Arnaf Jeen clinched gold in boys under 18 years event while Nada Radwan took silver in women's category.
Commercial Bank’s Qatar Masters 2020 will be held from March 5th to 8th at the new venue.
It was Scott's second Australian PGA title after romping to victory in 2013.
The 39-year-old headed into the tournament on the back of bitter disappointment at being on the losing side during last week's Presidents Cup to Tiger Woods' Americans at Royal Melbourne.
US player-captain Tiger Woods steered his team to a 16-14 victory over Ernie Els' Internationals in the 13th Presidents Cup in Melbourne.
The 43-year-old won all three matches he played, skipping only the Saturday fourballs and foursomes to focus solely on his captaincy duties.
But the big-time experience of the Americans came into play and they clinched a 16-14 victory in Melbourne for their 11th win in 13 editions of the biennial match play event.
With all 12 team members playing singles, 43-year-old Woods will be back to try and inspire the US to overturn a 10-8 deficit and inflict another demoralising defeat on the Internationals.
Internationals need another nine from Saturday's eight matches and the 12 singles on Sunday for a famous victory.
The 43-year-old Woods and world number four Thomas will tee off first on Thursday against Australian Marc Leishman and Chilean rookie Joaquin Niemann for Ernie Els’ International team.