Oosthuizen, looking as calm as the conditions, was at 11-under 129, breaking the 36-hole Open record first set by Nick Faldo in 1992 at Muirfield and matched by Brandt Snedeker in 2012 at Royal Lytham & St. Annes.
This will be the fifth time in the last nine rounds at a major that Oosthuizen, the 2010 Open champion at St. Andrews, has had at least a share of the lead. He was runner-up at the last two majors, to Phil Mickelson at the the PGA Championship in May and to Jon Rahm the U.S. Open last month.
"I'm not really going to think about the second spots," said Oosthuizen, when asked what he'll do differently this time. "I know my game is in a good place."