Cal O'Reilly was born on September 30, 1986 in Toronto, Ontario. He played his junior hockey in the Ontario Hockey League for the Windsor Spitfires and at the 2005 NHL Entry Draft he was selected in the fifth round, 150th overall by the Nashville Predators.
In 2008-09, O'Reilly appeared in 11 games for the Predators, scoring the first 3 goals of his career. October 28, 2011, O'Reilly time with the Predators organization would come to an end. He was traded to the Phoenix Coyotes for a fourth round draft pick.
On February 1, 2012, O'Reilly was picked up on waivers by the Pittsburgh Penguins. He would make his debut with the team three days later and suit in five more during the 2011-12 season.
On July 16, 2012, O'Reilly signed a two-year contract with Kontinental Hockey League team, Metallurg Magnitogorsk.
On November 19, 2013, O'Reilly signed a deal with the Utica Comets of the AHL. In 52 games for the club he recorded 45 points. His strong play was rewarded when the Comets' parent club, the Vancouver Canucks signed him to a deal in the summer of 2014.
On July 1, 2015, O'Reilly signed a two-year one-way contract with the Buffalo Sabres, uniting him with his brother Ryan.
O'Reilly would spend the 2016-17 season predominantly in the American Hockey League. First with the Rochester Americans and then the Toronto Marlies. He also appeared in 11 games for the Sabres, recording an assist.
In the summer of 2017, as an unrestricted free agent, O'Reilly signed with the Minnesota Wild.
