Oleksiak famously dead-heated for gold with USA's Simone Manuel at Rio 2016 to become Canada's youngest Olympic champion with Manuel the first African American swimmer to win an individual title at the Games.
Joining the four-time medallist from Rio are two-time reigning 100m backstroke world champion and Rio bronze medallist Kylie Masse, and Maggie MacNeil who stunned Olympic champion Sarah Sjostrom to take the 100m butterfly title at the Gwangju 2019 Worlds.
Backstroker Markus Thormeyer is the only man of the six who were either medallists or top Canadian finalists at the 2019 World Championships.
Thanks to her fifth place in the Gwangju 100 free, Ruck has been selected for that event with Oleksiak - who pulled out of the 100 - named for the 200 free in which she finished sixth.
Canada had its best ever World Championships in Gwangju, collecting two golds and six bronze medals.
In a Canadian Olympic Committee (COC) press release, Masse said, "It’s definitely a different way to be named to the Olympic team, but it’s still exciting. It’s been a challenging year so it’s nice to have a little bit of certainty in a time of so much uncertainty.
"I’m grateful that Swimming Canada and the COC have provisionally nominated me. I can’t wait to continue to work towards Tokyo to represent Canada."
Swimming Canada also announced that their Olympic trials would be switched from April to 24-28 May although there will be no automatic qualification from the Toronto event.
Instead, hopefuls will be invited to a further meet in Toronto from 21-23 June with the selectors choosing the team after that.
Its CEO Ahmed El-Awadi said, "The pandemic continues to dictate some of our decisions, and limit what we can and can’t do. While we want to keep as many options open as possible, we have to adapt to the constraints that are thrust upon us by the pandemic.
"What is being presented today allows us to have more options in an uncertain situation. It gives us greater ability to nominate the best teams to represent Canada, and it gives the athletes greater ability to train and compete to make Team Canada."