Montgomerie saw his chances of overhauling US Open champion Michael Campbell in the European Tour Order of Merit this weekend fade away as his three birdies were cancelled out by a bogey and a double bogey for a second consecutive par 72.
Oosthuizen, 29, had rocketed into the lead when he holed his second shot at the par-five second hole for only the fourth albatross in the history of the Masters.
Masters champion Mike Weir lies second on level par, with Open champion Ben Curtis one over and USPGA winner Shaun Micheel two strokes back.
American Kevin Sutherland hit a second successive five-under-par round of 65 to go a shot clear in second place.
English duo Ian Poulter (70) and Anthony Wall (68) and France's Christian Cevaer (71) are in a tie for second on three under par.
Before the weather intervened, Sergio Garcia had to climb up into one of the trees to play his second shot at the par-four 10th hole after his ball became lodged in the branches.
The first came on the par-five second hole Sunday, when Oosthuizen scored a rare double-eagle.
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Guan had a bogey at the par-3 second after his tee shot off the back of the green.
The best action Saturday was during the rain delay when Garcia was asked about the par-5 second hole.
Garcia, who started with a one-shot lead over Woods, was in the fairway on the par-5 second hole.
Els narrowly misses a long uphill putt with his second shot, but makes par from four feet with his third.
Scott eventually emerged victorious on the second playoff hole, the par-four 10th, fulfilling the dreams of the 15-year-old who watched Norman's infamous capitulation.
The American, who attended UCLA, made par on the second extra hole, the 10th, while Beljan missed a five foot putt to prolong the playoff.
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Woods began his final round in a share of eighth place on 13 under, and despite tripping up with a five at the par-four second, he amassed seven birdies to push Furyk all the way.
In later action, Woods battled to make the weekend on the cut line of level par after a second straight 70 with 23-year-old rookie Luke Guthrie leading the way on nine-under 131 after a superb seven-under 63.
Ireland's Harrington, bidding for a third straight major, ran up a quadruple bogey nine on the par-five second and did well to recovery his composure to finish with 73 for one-under, while Garcia, still seeking his first major carded a three-over 75.
While he birdied the second and third to improve to 20 under par, nearest challenger McIlroy followed his matching four on the long second with bogeys at two of the next three holes.
The 23-year-old, ranked second in the world, carded a level-par round of 72 on Thursday but produced a much improved performance during his second round.
Only a great recovery on the 17th maintained his advantage when he chipped to seven feet for par after a poor second shot.
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The Queenslander gave himself the chance to draw level with his second at the 449-yard par four but his birdie putt swung to the right of the cup.
Oosthuizen spent most of the round, after his three-under-par miracle on the second, protecting his position, whereas Watson didn't punch his way into a share of the lead until the 16th hole.
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Donald, who was on antibiotics, may have had an excuse in the freezing wet conditions on the West Course, and battled to a level-par 72 in his second round after a sorry 78 on the first day.
Still, that memo has yet to reach Aussie Adam Scott, who shot 69 to climb into second place at six-under par, or Marc Leishman, the other Aussie in contention, who shot even par to stay at five-under.
Woods carded a second consecutive five-under-par 67 to join overnight leader Nick Watney (70) on 10-under-par, with world number two Phil Mickelson returning a 66 to lie a shot behind alongside compatriot Ryan Moore and Spain's Alvaro Quiros.
The Englishman is currently tied for 10th on six-under par after posting a polished second round 68.
The Welshman carded a third-round 65 at Crans-sur-Sierre for a 13-under-par as he chases a second European Tour title.
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