No winner of The Claret Jug has ever gone on to lift another trophy on The European Tour a week later, but Oosthuizen still has that chance after adding a two under par 70 to his opening 67 at the Nordea Scandinavian Masters.
However Choi, whose only previous victory in Europe was the 2003 German Masters, is two ahead of the South African at halfway, with Swede Richard Johnson sandwiched between them and Argentina's Rafa Echenique tied with Oosthuizen for third..
Two weeks ago Choi switched to a bizarre-looking side-saddle putting method once used by the great Sam Snead.
But, after missed cuts at the John Deere Classic and then in The Open Championship at St Andrews, the 40 year old has gone back to a conventional style.
"I need more practice at it. These greens are very slopey," said Choi.
He eagled the long 15th - his sixth hole of the day - and birdied three of the next four to open a five stroke lead.
Oosthuizen had his misses on the greens, but has no plans for any drastic changes.
"You've got to do what you've got to do, but I think I'll stick to my routine," said the 27 year old.
Like Choi, Johnson is based in America and his only European Tour success was eight years ago - in Australia.
The 33 year old's bogey-free 66 was the low round of the day.
Johnson's compatriot and playing partner Jesper Parnevik, out of golf since February after the discovery of a career-threatening fractured vertebrae, quit after 11 holes of his second round.
That was longer than he feared he might last and Parnevik, five over at the time, will now continue the rehab which he hopes can make him a Tour regular again.
Italian Edoardo Molinari, who with a second place finish could knock Luke Donald out of the all-important top nine on The Ryder Cup table, is still well in touch at five under.
American Cup hopeful Dustin Johnson is only one further back after a 73 containing five birdies, six bogeys and seven pars.
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