One of them was Emma McKeon, who is now the most successful athlete in Commonwealth Games history.
McKeon added two more medals on Tuesday night for a career haul of 19 – the previous all-time record was 18.
But O’Callaghan chased her, and Shayna Jack, down on the Dolphins’ most dominant night yet in Birmingham.
Australians won six gold, six silver and five bronze medals on a glittering night.
O’Callaghan, her fellow 18-year-old Elizabeth Dekkers, Ariarne Titmus, para-swimmers Colin Pearse and Jasmine Greenwood, and the nation’s 4×100 mixed medley relay team, all triumphed.
The Dolphins now have 22 golds. With nine more on offer on the last night at the pool, they could yet surpass the swim team’s record of 28 golds at the Gold Coast Games four years ago.
O’Callaghan, who still has a year of school remaining, is now the world and Commonwealth champion.
But she said: “I have still got a long way to improve.
“This is just the beginning … I’m still learning.”
Jack, who won silver, and McKeon, who took bronze in the 100m free final, are astounded by a swimmer they consider a little sister.
“She’s incredible” McKeon gushed.
O’Callaghan now has four golds and one silver in Birmingham.
“Mollie, for me, is really special because we are training partners so I do have more investment in her,” Jack said.
“It’s a special moment for me and for Emma to see someone so young doing such an amazing job.”
Dekker, another emerging 18-year-old, was stunned with her 200m butterfly victory.
“It’s a bit of an unreal feeling, I never expected it,” she said.
Para-swimmers Pearse (men’s 100m butterfly S10) and Greenwood (women’s 200m individual medley SM10) joined the gold rush, as did Titmus, who headed an Australian sweep of the 800m freestyle with Kiah Melverton and Lani Pallister.
And Australia’s 4×100 mixed medley team – Kaylee McKeown, Zac Stubblety-Cook, Matt Temple and McKeon – surged to a win which delivered McKeon the all-time medal record.
The previous record for most Commonwealth medals, 18, was shared with Australian shooter Phillip Adams, English shooter Mick Gault and South African swimmer Chad le Clos.
The unassuming McKeon said she was more nervous watching her celebrity boyfriend Cody Simpson swim – he finished fifth in the 100 butterfly final.
“He will be happy with that,” McKeon said.
“It’s his first international experience … it’s just gaining intel.”
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