Nobody has ever lifted The Claret Jug and then won another European Tour title seven days later, but after a second round 70 at Bro Hof Slott, Oosthuizen is only two behind Korean Choi.
And that was a good result in the end for the 27 year old South African.
Two clear after birdies at the 11th and 12th - he resumed on the back nine - Oosthuizen then fell five adrift after going in the water at the short 17th while Choi had an eagle and three birdies around the turn.
But the 40 year old US PGA Tour regular, who for the time being has abandoned his experiment with the side-saddle putting stroke he used at St Andrews, then ran into problems on the undulating greens.
Choi, back with a conventional putter, three-putted from only ten feet at the fourth and after coming back with another birdie at the next he bogeyed the next two after missing from nine and seven feet.
A three footer for birdie then failed to find the target on the 454 yard eighth and after hitting a superb second to 12 feet at the long ninth he had to settle for birdie and not the second eagle he was hoping for.
With a 67 Choi posted a nine under aggregate of 135, while at seven under Oosthuizen was himself two in front of Australian left-hander Richard Green, who was only just starting his round.
Oosthuizen said: "It was very cold when we started and quite a bit windy.
"I could see a good round out there still, but I didn't putt so well and I'll try to sort that this afternoon."
Choi commented: "I am very happy. In the cross-wind control is very hard, but I hit the ball so good.
"These greens are very slopey and the side-saddle needs more practice."
American Dustin Johnson, joint overnight pacesetter with Green and Oosthuizen, produced a real mixed bag of a 73 to stand four under with Swede Peter Hanson.
Johnson, who a month ago led the US Open Championship by three and then shot a closing 82, had five birdies for the second day running, but after going bogey-free in his opening round he had six this time.
Like Choi and Oosthuizen, the two players ahead of him, Green played the back nine first and with birdies at the 13th and short 16th he went alongside the South African.
Northern Ireland's Michael Hoey, meanwhile, was up into fourth when he started with three successive birdies.
|1||JOHNSON, Richard S||SWE||18||-11|