McKeown on fire in Sydney as she warms up for Olympics
Australia's McKeown touched in 2mins 04.21sec at the Sydney Olympic Park Aquatic Centre to beat the field by seven seconds -- the second-fastest time this year behind American arch-rival Regan Smith and the ninth quickest ever.
It was at the same championships last year that McKeown set the current world record of 2:03.14, bettering Smith's previous mark.
McKeown also owns the best times in history over 50m and 100m -- the first woman to hold world records in all three backstroke disciplines.
Two-time 100m freestyle world champion O'Callaghan got the better of Olympic gold medallist McKeon as they also kickstarted preparations for Paris.
She hit the wall first in an eye-catching 52.82 with Australia's most decorated Olympian McKeon (53.70) relegated to third by Bronte Campbell (53.50).
Kyle Chalmers, the 2016 Olympic gold medallist and silver medallist in 2020, was upset in the 100m freestyle final with William Yang, who had back surgery last year, beating him to the wall in 48.48.
Meanwhile, Ariarne Titmus clocked a competitive 8:17.87 to win the women's 800m freestyle.
Titmus stunned Katie Ledecky to win the Tokyo Olympics 200 and 400m gold, but finished second behind the American great in the 800m.