O'Callaghan breaks record as Marchand adds more swimming gold
O'Callaghan beat Olympic champion Ariarne Titmus and Canada's Summer McIntosh to touch the wall in 1min, 52.85sec in Fukuoka, breaking a world record that had stood since 2009.
The 19-year-old injured her knee five weeks before the championships in Fukuoka and she said she was "really shook" to break the record.
"I couldn't really explain it in the moment," said O'Callaghan, who had fired a warning shot when she beat Titmus over 200m at the Australian trials last month.
"There were tears, there was happiness. It was very mixed emotions."
There was more glory for Marchand, who added the 200m butterfly title to the 400m individual medley gold he won in record-breaking time on the opening day.
Tunisia's Ahmed Hafnaoui took revenge on Australia's Sam Short to win the men's 800m freestyle, while China's Qin Haiyang picked up his second gold in the men's 50m breaststroke.
China then claimed another title when they won the mixed 4x100m medley relay.
O'Callaghan came out on top after another epic women's freestyle battle.
Titmus was looking to grab her second gold of the championships after winning the 400m in a world record time on the opening night.
She led for most of the race but O'Callaghan turned on the afterburners over the home straight to reel her in.
Titmus finished second in a personal-best time of 1:53.01, ahead of McIntosh on 1:53.65.
O'Callaghan's time beat the world record of 1:52.98 set by Italian Federica Pellegrini -- wearing a supersuit -- at the 2009 world championships.
"Heading into this, I just really wanted to have fun and enjoy it and try my best and try some new race plans," said O'Callaghan.
"To come out with a world record was really unexpected for me."
Marchand wins again
Marchand made headlines on Sunday when he beat Michael Phelps's legendary 400m IM world record, a mark that had stood since the 2008 Beijing Olympics.
He picked up his second title in the 200m butterfly, coming home in 1min, 52.43sec, ahead of Poland's Krzysztof Chmielewski on 1:53.62 and Japan's Tomoru Honda on 1:53.66.
Marchand was back in action just 40 minutes later, qualifying from the 200m individual medley with the fastest time.
"I have one more race, then relays with the team," said Marchand, who has dropped the 200m breaststroke from his programme.
"I'm just trying to enjoy the moment and keep going. It's been very good the past few days."
Hafnaoui came off second best to Short in the 400m freestyle final on Sunday, an event in which the Tunisian is the reigning Olympic champion.
He got the better of his Australian rival in the 800m, coming home in 7min, 37.00sec, ahead of Short on 7:37.76 and American Bobby Finke on 7:38.67.
Hafnaoui, Short and Germany's Lukas Martens were neck-and-neck for almost the entire race until Hafnaoui pulled away at the last.
"It's too much adrenaline when you see all these guys next to you battling for the gold," he said.
China endured a championships to forget last year in Budapest, picking up only one individual swimming gold medal.
Qin gave the Chinese team their third success so far this week in the 50m breaststroke, and his second individual title of the competition.
He finished in a time of 26.29sec, ahead of American Nic Fink on 26.59 and China's Sun Jiajun on 26.79.
China then won the mixed 4x100m medley relay in a time of 3min, 38.57sec, ahead of Australia on 3:39.03 and the United States on 3:40.19.