Mollie Callaghan wins world 100m freestyle title in new teen strike
Stay tunned with 24 News HD Android App
American veteran Lilly King added to her collection of gold medals when she won the women's 200m breaststroke.
O'Callaghan,a double Olympic relay gold medallist, edged the 28-year-old Swede by 0.13sec. American Torri Huske took bronze.
"It's absolutely weird at the moment to think that I am the world champion," said O'Callaghan.
O'Callaghan, who had already collected a world relay gold in Budapest, followed 17-year-olds David Popovici of Romania and Benedetta Pilato from Italy and 15-year-old Canadian Summer McIntosh in collecting individual gold at these championships.
O'Callaghan said she had suffered badly from pre-race nerves.
"It was bad, the worst ever," she said.
"I was panicking in my bed, having a little bit of a cramp in my leg, just feeling dizzy, feeling out of it, starting to panic, but I knew I had my teammates there... I guess that kind of uplifted me for the race."
It was Sjostrom's 16th World Championship medal but while she has eight golds, she has never won the 100m free. This was her fourth silver.
She also has world-championship and Olympic bronzes in the race.
After she missed a medal in the 100m breast on Tuesday, her college coach, Ray Looze, told American media that she was racing at "80 percent of what her capability is."
On Thursday, in a longer distance, she found reserves in on the last lap, coming from fifth place to grab victory in 2min 22.41sec. Australian Jenna Strauch was second at 0.63 with American Kate Douglass third.
Kristof Milak, the Hungarian who said after winning the 200m butterfly that the Duna "is my pool", strolled out for butterfly 100m semis like a lord strolling his estate.
Milak has struggled to catch American Olympic and world champion Caeleb Dressel in the 100m butterfly.
With the American heading home, the Hungarian star justified his aura of confidence by comfortably swimming the fastest time.
He finished in 50.14sec, 0.67sec quicker than Naoki Mizunuma of Japan.
In the 50m, another event that Dressel has dominated in recent seasons, Briton Benjamin Proud was fastest in the semis.