San Francisco Frontiers - O'Callaghan upstages Olympic champion Titmus as McKeown challenges world record

Kyle Chalmers was also in blistering form at the Australian championships, swimming a lightening 100m free to lay down a marker ahead of the Olympics.

O'Callaghan hit the wall on the Gold Coast in an eye-popping 1min 53.57sec, with an explosive last 50 pulling her away from Titmus, who touched second in 1:55.38.

"I'm always striving to go fast," said O'Callaghan, who also won the 100m title.

"The 200 is a difficult event to work out and this was the last time to get it worked out before trials."

Australia's Olympic trials are at Brisbane in June, with the 20-year-old potentially targeting six gold medals in Paris this summer.

All-conquering backstroke star McKeown will also be in the hunt for multiple Olympic titles.

She clocked the fastest 200m backstroke of the year ahead of American arch-rival Regan Smith by powering home in 2:03.84 to miss her world record by just seven-tenths of a second.

"I just wanted to go out hard and try and hang on," said McKeown, who was under world record pace at the halfway mark. "I definitely felt it in the last 35 metres."

McKeown also won the 50m backstroke, but skipped the 100m to focus on the 200m and 400m medley where she set among the fastest times ever.

The 22-year-old confirmed she will swim the 200 medley at the Olympic trials, along with the backstroke events, but not the 400 medley.

Chalmers, the 2016 Olympic champion who took silver in Tokyo behind American arch-rival Caeleb Dressel, blitzed to a 47.63 victory ahead of William Yang (48.20).

Since the last Olympics, China's Pan Zhanle has burst on the scene to challenge the old guard, setting a new world record of 46.80 this year.

He is in the only man faster than Chalmers this year.

"I really wanted to go under 48, so really, really happy with that," said Chalmers.

"I love racing, no matter where it is, when it is, I love getting myself up for a race. That's what keeps me motivated."

In other races, Japan veteran Daiya Seto took out the 400m medley in 4:10.44, comfortably beating reigning world champion Lewis Clareburt of New Zealand.

Both men were swimming on invitations, with Seto scoring revenge after Clareburt beat him into third at the February world titles in Doha.

Australia's most decorated Olympian Emma McKeon won the women's 100m butterfly in 56.58 ahead of Brianna Throssell (56.77).

L.Nguyen--SFF