Temperatures at Royal St George's were already pushing 25 degrees Celsius by 10am but, more significantly, the breeze had dropped to a barely a flutter.
As a result the R&A kept pin positions tucked away on most holes to maintain the difficulty level while the hardening bumpy fairways increased the likelihood of an unfortunate bounce kicking balls into the rough.
Oosthuizen, however, was afforded the luxury of being able to sit back and watch the day unfold as he was not due to tee off alongside 2020 US Open winner Collin Morikawa until 2.35pm.
It is 11 years to the day since the South African, who finished runner-up in the US PGA and US Open this season, lifted the Claret Jug at St Andrews.
But his one-shot lead is vulnerable, especially with another former winner Jordan Spieth in the group in front three shots back and still smarting from bogeying his last two holes.
If the 2017 champion needed any inspiration it was provided by compatriots Kevin Kisner and Brendan Steele.
Kisner, who usually shares a house with Spieth during Open week but has been prevented from doing so by Covid-19 bubbles, was first man out on his own and had four birdies and two bogeys through 15 holes.
Steele started even better by picking up three strokes in his first seven bogey-free holes.
Jon Rahm, who won his maiden major last month's US Open, tees off just before Spieth five shots back and will look to make a charge and exert some pressure on the top of the leaderboard.