The 2010 Open Champion, who has already recorded two runner-up finishes in Major Championships this year, made six birdies in a mistake free effort as golf's greatest links challenge returned after a year away due to the coronavirus pandemic.
Resurgent 2017 champion Jordan Spieth was at five under alongside fellow American Brian Harman, with another former winner in Stewart Cink, Major Champion Webb Simpson, Frenchman Benjamin Hebert, Canadian Mackenzie Hughes and South African Dylan Frittelli two off the lead.
Oosthuizen is second in the Race to Dubai Rankings in partnership with Rolex following his second place finishes behind Phil Mickelson at the US PGA Championship and Jon Rahm at the U.S. Open but is looking for European Tour win number ten this week.
"Yeah, probably in my mind the perfect round I could have played," he said. "I didn't make many mistakes. When I had good opportunities for birdie, I made the putts. So yeah, just a very good solid round."
His round started with seven straight pars before he sandwiched a 20 footer at the ninth with birdies at the eighth and tenth in a hat-trick of gains.
Back to back birdies at the 13th and 14th from inside ten feet put Oosthuizen in a share of the lead with Hughes and Spieth before he put his tee shot to six feet at the 16th.
A brilliant up-and-down for par after finding a fairway bunker at the last then secured the 38-year-old the lowest round in a Major on the 2021 Race to Dubai.
"I think I probably would have taken seven pars again," he said. "I've learnt over the years playing Major Championships that patience is the key thing and even if you make bogeys, know that a lot of people are going to make bogeys."
It was clear that Oosthuizen is not holding on to the disappointment of his near-misses at Kiawah Island and Torrey Pines, and he admitted after his round that he only needed a few days before he was able to forget about them.
"Yeah, it depends if you lost it or someone else beat you," said Oosthuizen, who now has six runner-up finishes at Majors - including at The Open in 2015.
"I think in both of those I was beaten by better golf at the end there. It takes a little while but it's sort of, you have to get over it quickly, otherwise it's going to hold you back to perform again.
Probably in my mind the perfect round I could have played
"I tried to take a few days and just try and forget about it and see if I can get myself ready for the next one."
But the knowledge that he can still compete in Majors, coupled with past success in this tournament in particular, also helps feed his confidence and frame of mind - which is something he brought to the opening day.
"It gives me confidence going into Majors knowing that I'm still competing in them and I've still got chances of winning," he explained.
"But yeah, once the week starts, I need to get that out of my mind and just focus on every round and every shot. But it definitely puts me in a better frame of mind going into the week."
Spieth got off to a tentative start with three pars and a bogey in his first four holes before kickstarting his momentum with a birdie putt at the fifth before adding three more at the sixth, seventh and eighth.
A run of six straight pars followed before back to back birdies at the 15th and 16th and a closing par saw the former World Number One close out an opening round of 65.
Harman made a remarkable start with birdies on the first, second, third and fifth, and he was five under after eight holes before dropping a first shot of the day on the ninth.
He bounced back from another bogey on the 12th with a birdie on the 13th and a closing gain had him within one.
Hebert is playing in just his third Open but has some links pedigree having lost a play-off at the 2019 Scottish Open, and he carded birdies on the third, fourth, 12th and 14th in a bogey free effort.
Cink was also bogey free, while Hughes made his only bogey of the day on the 15th to drop out of a share of second, with Frittelli and Simpson also both dropping a single stroke.
Home favourites Tommy Fleetwood, Justin Rose, Jack Senior, Andy Sullivan and Danny Willett, birthday boy Marcel Siem, Americans Collin Morikawa and Scottie Scheffler, South African Justin Harding and South Korean Byeong-hun An were at three under.