Oosthuizen began the day two shots off the lead and briefly moved to outright leader prior to Lombard's recovery.
He made a double-bogey seven, while both Lombard and Oosthuizen made birdies to be two ahead of the Belgian.
The 24-year-old said: "I'm still in the lead".
On a day when sub-70 scores were a rarity, Oliver Wilson of England soared up the leaderboard with a five-under-par 67 to share fourth place on eight-under with Sweden's Marcus Kinhult (70).
Successive birdies at 10 and 11 put Detry in a three-way tie with Lombard and Oosthuizen before a magnificent 190-yard iron shot from a fairway bunker set up a birdie at the 13th to put him in the lead on his own.
"My body is feeling great, I had a great rest last night and I felt great this morning so no issues there". But, all in all, I'm still in the lead.
"I feel like I've matured enough to handle it and hopefully going forward do one better tomorrow", said Lombard.
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"I don't want to sound cocky but it's been a long time coming". I've made big strides in how I do things on and off the golf course. "It's nice to get some feedback and some results from the hard work".
Lombard recovered with four birdies in the space of seven holes and despite another bogey at 16, he is the man to catch on 11 under at Gary Player Country Club.
He responded with birdies at the next two holes, but he found sand off the tee at the last and came up short of the green with his second, from where a clumsy chip ran 15 feet past the flag and he would have to settle for a two-putt bogey and a round of 69 that could have been a few lower.
Oosthuizen is hot on the heels of his compatriot after carding a one-under 71, two bogeys costing him the lead on moving day.
"There were lots of three and four iron approach shots, so I am very pleased with the comeback".
"Playing with two legends of the game, it was nervy, but I settled down quickly with a birdie on the second and it was just all really good vibes from there".
All three were in a tie for the lead after Lombard - who was playing alongside defending champion Lee Westwood and South African legend Ernie Els - went eagle-birdie after the turn, but his rivals started heading in the wrong direction as he took control.