Rising star Mollie O’Callaghan upstaged fellow Australian Ariarne Titmus to clinch the Olympic 200m freestyle gold on Monday as Canadian prodigy Summer McIntosh blitzed the field to be crowned 400m medley champion.On a bumper night of medal action at La Defense Arena, Romanian teenager David Popovici won a frantic men’s 200m freestyle and Italy’s Thomas Ceccon the 100m backstroke. South Africa’s Tatjana Smith rounded out a golden evening with a thrilling victory in the women’s 100m breaststroke. O’Callaghan is known for her dynamic finishing power and that came to the fore once again. Hong Kong’s Siobhan Haughey led through 150 metres with Titmus on her heels before O’Callaghan turned on the afterburners to bring it home in an Olympic record 1:53.27.Defending champion and world record holder Titmus, who won the 400m title on Saturday, was second in 1:53.81 and Haughey third. “It’s such an honour to be with everyone, and compete against Arnie. She races like an absolute beast,” said O’Callaghan, who shares the same coach as Titmus. “It’s an honour to train alongside her and have such a great team around us.”It was the 20-year-old’s fourth Olympic gold medal, but her first in an individual event after making her mark as a relay swimmer in Tokyo. Since then she has blossomed, winning the 100m and 200m at the 2023 world championships and smashing Italian great Federica Pellegrini’s 14-year-old 200m world record before Titmus took it away in June.McIntosh also burnished her already glowing reputation with a dominant 400m medley. The 17-year-old world record holder powered to the wall in 4:27.71 – the fourth-fastest time ever – to outpace American pair Katie Grimes (4:33.40) and Emma Weyant (4:34.93). It was the Toronto swimmer’s second Paris medal after taking silver behind Titmus in the 400m free. She is also favourite in the 200 medley and a serious contender in the 200m butterfly as she hovers on the edge of superstardom.McIntosh, who made her Olympic debut in Tokyo as a 14-year-old, lowered her own world record in May when she clocked 4:24.38 and was always in charge. “Very happy to get the job done, to stand on top of the podium and get the gold medal,” she said. “I’m really just trying to soak up every single moment. “I mean, it’s always just about having fun, and pushing my body to its limits,” she added.Popovici made no mistake in the 200m freestyle, winning by a fingertip from Britain’s Matthew Richards. He had to work hard with Germany’s Lukas Maertens, the 400m champion, surging through the opening 150 metres in front. But the Romanian came home hard to touch in 1:44.72, just 0.02 ahead of Richards with American Luke Hobson third and Maertens fading to fifth.“It’s beautiful, and it’s a dream come true,” said Popovici, who burst on the scene by winning the 100/200m freestyle double at the 2022 world championship. “I don’t think words can describe it. It’s so simple yet so beautiful.”Smith went one better than her Tokyo silver in winning the women’s 100m breaststroke final, finishing strongly to pip China’s Tang Qianting and Ireland’s Mona McSharry. American world record holder Lilly King was fifth.Italian world record holder Thomas Ceccon proved too strong in the men’s 100m backstroke, reeling in China’s Xu Jiayu to hit the wall first in 52.00. Xu was second and American Ryan Murphy, the Rio gold medallist, third. In semi-final action, Regan Smith seized a narrow edge over Kaylee McKeown in their fierce battle for the Olympic backstroke gold medals.The pair have a storied rivalry that has ebbed and flowed, with world records changing hands between them. American world record holder Smith progressed to the 100m final in 57.97 with Australian defending champion McKeown just 0.02 behind. Fellow American Katharine Berkoff was third fastest ahead of Australia’s Iona Anderson.