Sam O'Reilly was born in Toronto, Ontario, Canada.
O'Reilly played for London in the Jr.B league and tallied 26 goals and 37 assists in only 35 games. He was drafted by the London Knights of the OHL with the #37 pick of the 2022 draft. He played in five games during the season for the Knights and was held scoreless.
In 2023-2024, O'Reilly established himself as one of the better players on the Knights. He played in 68 games and tallied 20 goals and 36 assists.