"She was laughing and then sank like a stone. ... Everyone froze," said ABC publicist Amy Astley. As the show cut to a commercial, Osmond started to rise and was helped by others, Astley said.
When the show resumed after the break, host Tom Bergeron told the studio audience and TV viewers that Osmond had fainted but was fine.
"I want to just quote her exactly. ... The first words out of her mouth when she saw us all leaning over her were, 'Oh, crap,'" Bergeron said. A backstage camera showed Osmond smiling and nodding.
The singer, who turned 48 on Oct. 13, was well enough to take part in the next segment, in which she and Roberts learned that judges Carrie Ann Inaba, Bruno Tonioli and Len Goodman had each awarded them seven out of 10 possible points.
Osmond explained on-air that "once in a while that happens to me when I get winded. I stop breathing." She punctuated her remark with a giggle. She was expected to undergo a medical examination as a precaution, ABC's Astley said.
The hit series pairs celebrities with professional dancers and is drawing more than 35 million viewers for its dance-off and results shows. The show puts its stars through rigorous dance routines that test many of them and proved too much for at least one.
Vincent Pastore, who played a tough-guy mobster in the early years of "The Sopranos," dropped out on last season's show in February after a week of training.
"I didn't realize just how physically demanding it would be for me. Unable to put forth my best effort, I felt it appropriate to step aside and give someone else the opportunity," Pastore, 60, said at the time. His replacement was John Ratzenberger of "Cheers."
Other contestants in the current edition include racing champion Helio Castroneves, billionaire Mark Cuban and Jennie Garth of "Beverly Hills 90210" fame.