K.J. Choi shot a 5-under 67 for a one-shot lead after two rounds of the Scandinavian Masters on Friday.
Choi had six birdies and an eagle and three bogeys to reach 9-under 135, one stroke ahead of Richard Johnson of Sweden, who had a 66.
Teeing off early in windy conditions at Bro Hof Slott, Oosthuizen had a double bogey at the 150-yard, par-3 17th after landing in the water.
The South African had three birdies and another bogey on his first nine holes, for a par 36, which was four shots off his performance on Thursday.
"I didn't putt as well today, I missed a few," Oosthuizen said. "I'll have a nice rest this afternoon. It should be great tomorrow."
Choi had an impressive start with an eagle on the 15th, as well as birdies at the 13th, 16th and 17th, for a 5-under 31 on his first nine.
"From the first hole I felt that my shot was there, my rhythm was good. I was hitting my driver, my irons very well," Choi said. "My putting was going good, too, so I felt a lot of confidence going into the first few holes."
Johnson had a bogey-free round in front of his home crowd put him 8 under after 36 holes.
Green was three shots off the lead after a 71, including consecutive bogeys at Nos. 4 and 5.