Scott, 21, owned a piece of the lead until a bogey at the last saw him finish with a 66 for a three-day total of 16-under-par 197. He was followed by two-time Masters champion Jos Maria Olazbal, whose 7-under 64 earned him a share of third place at minus 15.
Bjornstad began the day six strokes off the lead but quickly made his presence felt with four birdies and an eagle over a span of five holes, starting at the second. He rolled in a 15-foot birdie putt at the par-3 8th for his third 2 of the round, then saved par out of bunker at the 9th to cap a sizzling front-side 28.
After spinning a wedge back to two feet for birdie at the 10th, Bjornstad found a bunker with his second shot at the par-5 12th and blasted out to tap-in range for a birdie that took him to 16-under.
The Norwegian's run at the magic number 59 came screeching to a halt at the 13th, where he failed to convert a birdie putt after his approach stopped within three feet of the hole. Bjornstad also missed a 10-foot try at the 14th but did manage to sink a 25-footer for birdie at 16 to post a new course record at Hong Kong Golf Club.
'Everything just went my way. I hit almost every shot good,' said Bjornstad, who is seeking his first professional victory. 'If I had made the putts, it would have been real good. But I can't complain about a 61.'
Bjornstad, the Norwegian Amateur champion in 1996, was a member of the European Tour in 1999 but lost his playing privileges after missing 13 cuts in 20 starts. His best showing that year was a tie for 13th at the Heineken Classic.
Since winning his card back at Q-School prior to the 2001 season, Bjornstad has recorded three top-10s, including a career-best tie for fourth at the Argentina Open. His improved play helped him finish 83rd on the Order of Merit, giving him exempt status on the 2002 tour.
Scott, the first-round leader after shooting 64 on Thursday, turned a poor opening drive into a bogey and slipped to 10-under-par. But he birdied half of his next 14 holes, culminating in a chip-in birdie at the 15th, to join Bjornstad, who was already in the clubhouse, at 17-under.
Although Scott missed the fairway at the 18th, his approach easily carried the water fronting the green and landed on the front edge of the putting surface. He left his first putt five feet short, however, and his bid to save par slid by the left edge of the cup.
'I moved myself into contention. It was a good job done,' Scott said. 'I played much better than yesterday and hopefully, I can keep it going. It would have been nice to have tied for the lead but a lot of stuff can happen out there.'
Scott, who turned pro in mid-2000 after an outstanding amateur career, reeled in his breakthrough win in his first start of 2001 at the Alfred Dunhill Championship in South Africa.
Olazabal, who captured his 19th European Tour title at the French Open in May, put himself in position to seriously contend for No. 20 with a bogey-free round made up of five birdies and an eagle.
The 35-year-old Spaniard, a runner-up at the BMW Asian Open in Taiwan a week ago, admitted he'll have to do better than his average of 31 putts per round this week if he wants to come out on top Sunday.
'Its going to be a matter of making putts and that is going to be a tough task for me,' he said. 'The grains break one way, then another way and finally you really don't know where to hit it. It is tough to get the pace right. That has been my problem.'
Also at 15-under was England's Mark Foster, who finished first on the 2001 Challenge Tour money list. He turned in a six-birdie, two-bogey 67.
Welshman Mark Pilkington carded a flawless 62 to finish three strokes off the pace with Swedish players Anders Forsbrand and Carl Pettersson, who had matching 68s.
Taiwan's Yeh Wei-Tze and Zaw Moe of Myanmar were 1-2 after Friday's second round but fell back into a tie for eighth place at 12-under 201 with four others, most notably defending champion Simon Dyson of England.
The Hong Kong Open, which began in 1959, is being co-sanctioned by the European Tour and Davidoff (Asian PGA) Tour for the first time. This is the second event to count toward the 2002 European Tour Order of Merit and the season-ending tournament on the 2001 Davidoff Tour schedule.
Thailand's Thongchai Jaidee, who also finished second last week in Taiwan, has all but locked up the 2001 Davidoff Tour Order of Merit title. He shot a third-round 67 for a share of 26th place while his main competition, Charlie Wi of Korea, struggled to a 3-over 74 and dropped to 78th.
For Wi to take the money title he will need to finish in the top five and Jaidee will have to take the same sort of tumble that Wi suffered Saturday.
Full-field scores from the Hong Kong Open