Two American veterans, Lilly King in the women's 200m breaststroke and Ryan Murphy in the men's 200m backstroke, then won breakthrough gold medals before the US men finished the evening by winning the mens 200m freestyle relay.
Australian, Zac Stubblety-Cook, who took the men's 200m breaststroke, said there were reasons this had been a youthful competition.
"The year after the Olympics is a bit of a different feel," the 23-year-old said. "The young bloods are coming through."
He pointed out that in his event "two of us from the Olympics were in the final tonight."
The 18-year-old O'Callaghan, followed 17-year-olds David Popovici of Romania and Benedetta Pilato from Italy and 15-year-old Canadian Summer McIntosh in collecting individual gold at these championships.
O'Callaghan edged the 28-year-old world record holder Sarah Sjostrom of Sweden by 0.13sec. American Torri Huske took bronze.
At 18 years and 82 days, O'Callaghan became the youngest winner of the 100m freestyle since 1991, when Nicole Haislett of the United States won the title at 18 years and 22 days.
O'Callaghan said she had suffered badly from pre-race nerves.
"The worst ever," she said.
"I was panicking in my bed, having a little bit of a cramp in my leg, just feeling dizzy, feeling out of it, starting to panic, but I knew I had my teammates there... I guess that kind of uplifted me for the race."
King, who has dominated the 50m and 100m breaststroke, grabbed her first gold at 200m, the longest breaststroke distance.
"It's really nice to be able to complete the set, I guess I'm a distance swimmer now," said King.
'Mentor'
Murphy ended a long streak of duller coloured medals when he won the men's 200m backstroke.
Since grabbing two individual Olympic
Read more on beinsports.com
beinsports.com
