The 2010 Open champion shot seven birdies during his third round yesterday to hold a one-shot advantage over England’s Marcus Armitage, who raced through the field with a stunning 62 taking him to 14 under.
Oosthuizen could become the first player to successfully defend this title since fellow South African Trevor Immelman 16 years ago.
The 37-year-old, who is chasing a tenth European Tour victory, holed a 15-footer on the second, made another birdie on the third, and completed his hat-trick with the help of a delicate chip on the fourth.
Oosthuizen turned in 31 after a seven-foot putt at the ninth and added birdies at 11, 13 and 14 on the back nine.
Salford-born Armitage clearly has a taste for the Firethorn Course at Randpark Golf Club having carded a bogey-free 65 on day one.
Armitage, who regained his card at the Qualifying School having first made his way on to the European Tour in 2016, climbed 42 places with a round of two eagles, six birdies and only a single dropped shot.
The 32-year-old’s effort would have been a course record had preferred lies not been in place. “That’s why we play golf for those days, it was just magic,” said Armitage. “I holed a nice little wedge on three and it just went from there.”
Jaco Ahlers is one shot adrift of Armitage after making six birdies and three bogeys in a round of 68.
A five-strong group on 12 under contains England’s Andy Sullivan, who carded a round of 67, and a quartet of South Africans – Jayden Schaper, Branden Grace, Jacques Blaauw and Martin Rohwer.
Schaper was the overnight leader and the 18-year-old amateur stayed in the hunt with a two-under-par 69.
Scottish pair Marc Warren and Connor Syme are not out of the running yet either. Both are handily-placed on ten under after Warren, inset, carded six birdies in a 65 and Syme posted a 69 that contained five birdies but also three bogeys.
At the Sony Open at the Waialae Country Club in Honolulu, meanwhile, Russell Knox moved himself into contention with a second-round 65.
The 34-year-old hit six birdies and a bogey to move to within a shot of leaders Brendan Steele and Cameron Davis.
Steele birdied five of his last six holes – the run ruined by a double bogey on the sixth – while Davis made five gains and a bogey as both players recorded 66s during day two in Hawaii. The pair hold a one-shot lead over a group of nine players which, as well as Knox, includes major winner Keegan Bradley and overnight leader Collin Morikawa.
Bradley had five birdies in a 66 while fellow American Morikawa birdied the last two for a level-par 70.
Elsewhere, Wade Ormsby continued his fine run at the Hong Kong Open with a 4-under 66 to take a two-shot lead. Ormsby, who has led throughout, recorded an eagle, three birdies and a bogey in the third round for 13-under 197 overall.