Oosthuizen, 29, shook off two early bogeys on a sunny, windy afternoon at Redstone. He reeled off four birdies in a row on the back nine to surge ahead.
“It’s a great leaderboard behind me,” Oosthuizen said. “It’s going to be tough, but I feel like I’m ready for it.”
Mahan had 65, rebounding from a bogey on No. 9 with four birdies on the back nine.
Carl Pettersson (67) and Brian Davis (69) were three strokes back at 14 under. James Driscoll (71) was alone at 12 under. Defending champion Phil Mickelson (70), Keegan Bradley (69) and Ryan Palmer (66) were 11 under.
Three-time major champion Ernie Els, who must win to qualify for the Masters Tournament next week, was in a group at 8-under. Els hasn’t missed the Masters since 1993 and knew that his chances of winning Sunday are remote.
“I needed to get to 10 or 11 under to really have a shot,” Els said. “I need a 62 or 63. It’s tough to do on a Sunday, but you might as well give it a go.”
The tournament’s schedule was pushed back by a thunderstorm Thursday, and 70 players resumed their second rounds Saturday morning.
Oosthuizen completed a 66 to move to 11 under, one behind second-round leader Jeff Maggert. Oosthuizen started his third round with a tee shot into a fairway bunker, leading to a bogey, then misjudged the wind off the tee on No. 2 and bogeyed again.
The 2010 British Open winner sank an 11-foot birdie putt on No. 6 to get going, then made birdies at Nos. 8 and 9 to make the turn at 12 under.
“After that, I felt really comfortable,” Oosthuizen said. “I made really good swings after that.”
Mahan made four birdies in six holes to climb up the leaderboard. He holed a 34-footer on No. 6 and a 15-footer on No. 7. The winner of the Match Play Championship this year, Mahan never considered skipping Houston to go straight to Augusta.
Mahan has three top-10 finishes in five previous starts here since 2007.
“I thought about it for about a second,” Mahan said.
Ninety players made the 36-hole cut at 2 under or better. It was the most players to survive the 36-hole cut in a non-major since 91 made it at the 1981 Travelers Championship.
Another cut was made after the third round, leaving 70 players in the field for today.