O'Reilly was picked by the Boston Bruins in the first round as the 14th pick overall in the 1971 NHL Amateur Draft. O'Reilly spent his entire career in Boston, serving as the captain of the Bruins during the 1983–84 and 1984–85 seasons before his retirement. The Bruins retired his No. 24 on October 24, 2002. O'Reilly was known for being a tough player, racking up over 200 penalty minutes in five consecutive seasons, and earning for himself the nickname "Bloody O'Reilly" in the press. His teammate, Phil Esposito, dubbed O'Reilly "Taz" in reference to the Tasmanian Devil cartoon character for O'Reilly's reckless, hard driving style of play. He was very protective of his teammates. When the Bruins retired O'Reilly's No. 24, Ray Bourque noted that O'Reilly's banner "hangs next to mine, protecting me again." On top of his physical presence, he also had a decent scoring touch, highlighted by his 29-goal, 90-point season in 1977–78. He added to that with a 77-point effort the following campaign. He had 211 and 205 minutes in penalties in those seasons respectively, displaying an excellent balance of grit and scoring. He finished his 13-year career with 204 goals, 402 assists for 606 points, a +212 plus/minus and 2,095 minutes in penalties. In the infamous December 23, 1979, incident at Madison Square Garden, during a post-game scrum, a New York Rangers fan rolled up a program tightly and smacked Stan Jonathan in the face drawing blood, then stole his stick and wielded it like a weapon. O'Reilly scaled the glass and charged into the stands. His teammates followed when other fans tried to intervene. O'Reilly was suspended eight games for his part in the brawl. He became the replacement head coach of the Bruins during the 1986–87 NHL season and kept his job until 1989, when he left to care for, and spend more time with, his son who was seriously ill with liver disease. In that time, he took the Bruins to the Stanley Cup finals in 1988, where they were defeated by the Wayne Gretzky-led Edmonton Oilers. O'Reilly also was an assistant coach for the Rangers for the two seasons prior to the lockout.
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In the Adam Sandler movie Happy Gilmore, O'Reilly is mentioned as Happy Gilmore's favorite hockey player when growing up due to his tough style of play. O'Reilly has stated his favorite player who played for the Bruins is Milan Lucic, also born on June 7. He was inducted into the Oshawa Sports Hall of Fame in 1990.
