After forgoing his senior year of college basketball, Olynyk was selected with the 13th overall pick by the Dallas Mavericks in the 2013 NBA draft, before being immediately traded to the Boston Celtics.

He and fellow future NBA player Cory Joseph were both on a Scarborough Blues club team that rarely lost in the late 1990s and early 2000s.

In order to improve his game and get stronger, Gonzaga and Olynyk agreed that he would redshirt his junior year (2011–12), meaning he would practice with the team, but not play.

[5] Following the 2012–13 season he opted for the NBA draft, thereby forgoing his senior year of eligibility (though by that time he had already received his bachelor's degree in accounting).

[12] On January 22, 2015, he injured his ankle after landing on the foot of Thomas Robinson in the fourth quarter of the Celtics' 90–89 win over the Portland Trail Blazers.

[13] Olynyk missed the Celtics' 2015–16 season opener against the Philadelphia 76ers on October 28 after being suspended for his role in Kevin Love's left shoulder injury during the 2015 playoffs.

[17] Olynyk missed the first six games of the 2016–17 season after undergoing right shoulder surgery in May 2016, and subsequently spent a day with the Maine Red Claws of the NBA Development League in early November.

[18][non-primary source needed] He made his season debut for the Celtics on November 9, 2016, playing 25 minutes and scoring two points in a 118–93 loss to the Washington Wizards.

[28] On February 10, 2020, Olynyk recorded a double-double, which was 12 points and a new career-high 11 assists, along with six rebounds, one steal and one block in a 113–101 win against the Golden State Warriors.

On March 25, 2021, Olynyk, Avery Bradley, and a 2022 draft pick swap were traded to the Houston Rockets in exchange for Victor Oladipo.

[30] Olynyk made his debut in a win over the Minnesota Timberwolves on March 27, recording 16 points, four rebounds, and four assists in 25 minutes.

[35] Two days later, the injury was diagnosed as a grade 2 medial collateral ligament (MCL) sprain, ruling Olynyk out of action for at least six weeks.

[40] On February 6, 2025, the Raptors traded Olynyk, Bruce Brown and multiple draft picks to the New Orleans Pelicans in exchange for Brandon Ingram.

[45] Olynyk's father, Ken, was head men's basketball coach at the University of Toronto from 1989 to 2002 and the Canadian junior men's national team from 1983 to 1996, notably cutting future Canadian basketball icon Steve Nash from the junior national team.