Spaniard Rafael Cabrera-Bello, Northern Ireland's Michael Hoey and Austrian Markus Brier joined Oosthuizen atop the leaderboard at six-under 66 at one of the European Tour's marquee championships.
Play rotates over famed St. Andrews, Carnoustie and Kingsbarns over the first three rounds as professionals compete with amateurs. The final round will be at St. Andrews, the home of golf.
Oosthuizen and Cabrera-Bello teed it up at Kingsbarns on Thursday, while Hoey and Brier played at St. Andrews.
Several of the world's best are in the field this week, first among them, the world No. 1 Luke Donald. The Englishman posted a three-under 69 at Kingsbarns and is tied for 22nd.
World No. 2 Lee Westwood shot a four-under 68 at Kingsbarns, as did defending champion and No. 6 player Martin Kaymer, and No. 3, Rory McIlroy rebounded from some early woes with a two-under 70 at the same course.
Oosthuizen put aside the disappointment of losing his spot on the International Presidents Cup team, then not getting picked by captain Greg Norman to share the first-round lead.
He began on the 10th tee Thursday and parred his first three holes. Oosthuizen, who cruised to his Open Championship title at St. Andrews last year, broke into red figures with a birdie at the par-three 13th.
The South African collected his next birdie at the next par three, the 15th. Oosthuizen finished his front nine with two birdies and a bogey to make the turn at three-under par.
Oosthuizen rattled off back-to-back birdies at the second and third, then got to six-under par thanks to a six-footer for birdie at No. 9.
Cabrera-Bello parlayed a brilliant first nine into his piece of first. He recorded an eagle at the par-five third and three birdies to turn in five- under 31.
The Spaniard posted two birdies and a bogey in his first three holes on the back nine. A tap-in birdie at the 16th left him alone in the lead at seven- under par, but a bogey at the last dropped him into a share of first.
Hoey, who won this year's Madeira Islands Open, flew out of the gate with back-to-back birdies at one and two. He added birdies at five and nine to get in the mix at four-under par.
Hoey dropped shots at the 11th and 12th holes, but got the strokes back with consecutive birdies at 13 and 14. He was four-under for the round, but joined the lead with his play at the last.
The Northern Irishman hammered his drive to the fringe as 18 was playing downwind. He had 40 feet for eagle from the fringe and holed the putt to match the leaders.
"I soon as I hit it, it came off perfect," Hoey said of his eagle. "Tracked all the way and made it. It was great."
Brier started on 10 and birdied two of his first three holes. He drove into a fairway bunker at 16 and three-putted for a double-bogey, but that was the last of his first-round mistakes.
Brier birdied 18 and continued the momentum around the turn. His approach hit the stick at one and set up an easy birdie. Brier rolled in a 15-footer for birdie at two and the Austrian was three-under par.
He drained a six-foot birdie putt at the fourth and two-putted the par-five fifth for birdie. Brier was now five-under and closed his round with a birdie at nine to become a joint leader.
"Played solid," said Brier, who is playing on a sponsor's invitation. "It's a very good first day. I'm happy to be here on an invite, but there's still three days to go."
NOTES: Oosthuizen and Cabrera-Bello will play Carnoustie on Friday, while Brier and Hoey are at Kingsbarns...Colin Montgomerie, Padraig Harrington and newly turned professional Tom Lewis were among the players tied for 11th at four-under par...Ernie Els shot an even-par 72 and is tied for 75th.