Australia's red-headed rocket Mollie O'Callaghan toppled defending champion Ariarne Titmus to claim gold in the women's 200m freestyle in an Olympic record at the Paris pool on Monday in a duel of clubmates.

FULL RESULTS

Prepared by the same coach Dean Boxall and training at the same suburban Brisbane pool, O'Callaghan and Titmus both bided their time in the race as Hong Kong's Siobhan Haughey set a cracking early pace at La Defense Arena.

O'Callaghan was third behind Haughey and Titmus at the last change, but threw down the hammer with a 27.98-second split in the final lap to overhaul them both in a time of 1:53.27.

That was a cool 0.54 seconds ahead of Titmus, with Haughey taking bronze after exhausting all reserves.

"It’s such an honor to be with everyone, and compete against Arnie. She is an absolute gun," said O'Callaghan.

"She races like an absolute beast. And it’s an honor to train alongside her and have such a great team around us."

O'Callaghan's win came only weeks after Titmus swiped her world record at Olympic trials, a psychological blow before the Games that left her in tears.

It meant little in the end, however, as she stood beaming with the gold medal around her neck, having convincingly beaten one of Australia's great swimmers for her first Olympic gold.

"I’m always striving for more and I always put a lot of pressure on myself. My expectations are very high. That was an amazing race. I’m always wanting that little bit more," she said.

"To be honest, I did it for the country, I didn’t do it for myself. I’m racing for all these people. I just had to put it behind me. Less pressure now, I get to swim freely."