Phelps won by almost two seconds at the Rio Games on Thursday to secure his 22nd Olympic gold medal.
The race was billed as a battle between Phelps and fellow American Ryan Lochte, but the latter ended up fifth.
Manuel and Oleksiak upstaged more fancied rivals in the women's 100m freestyle final, sharing gold ahead of Swedish star Sarah Sjostrom.
Ryan Murphy and Rie Kaneto were also among the winners, while world-record holder Missy Franklin missed the final of the 200m backstroke.
RACE OF THE DAY
It was meant to be a battle between Phelps and Lochte, with the former having won the 200m IM at every Games since 2004.
That never looked in doubt, but Lochte – a medallist in the event at each of those Olympics – was unable to challenge.
Phelps won by 1.95 seconds from Japan's Kosuke Hagino and China's Wang Shun.
Manuel and Oleksiak broke the Olympic record, touching in 52.70secs in a 100m freestyle dead heat.
American superstar Katie Ledecky set one of her own earlier in the day in the heats of the 800m freestyle.
"As I kid I wanted to do something that nobody had ever done before. I'm enjoying it," - Phelps said after yet another gold medal.
"I felt like David facing Goliath but I didn't have any stones in my pocket," - Franklin tries to explain missing the 200m backstroke final.
"My first gold medal, at my first Olympics, is kind of a surprise to me. I never thought I would be in this position but I'm so blessed and honoured to be on the medal stand. All my hard work has paid off and I am really happy about it," - Manuel was thrilled with her win.
"This is amazing, to tie for a gold. I never thought I'd win a gold. She [Manuel] deserves it as much as me. It means so much," - Oleksiak, a gold medallist at the age of 16, said.
More Phelps and more Ledecky. Phelps will need to beat the likes of Chad Le Clos and Laszlo Cseh in the men's 100m butterfly to win gold number 23. Ledecky may yet win a fourth gold in Rio in the 800m freestyle, with a potential world record to follow.