After being acquired from the Buffalo Sabres in a trade on July 1, 2018, O'Reilly led the Blues in scoring during the regular season (77 points; 28 goals, 49 assists), then won the Conn Smythe Trophy after helping St. Louis defeat the Boston Bruins in a seven-game Stanley Cup Final for its first championship since entering the NHL in 1967. He became the first non-goalie in NHL history to win the Conn Smythe in his first season after being traded. He had 23 points (eight goals, 15 assists) in 26 playoff games.

O'Reilly's first season with the Blues was one for the record books.

O'Reilly's first season with the Blues was one for the record books.

After being acquired from the Buffalo Sabres in a trade on July 1, 2018, O'Reilly led the Blues in scoring during the regular season (77 points; 28 goals, 49 assists), then won the Conn Smythe Trophy after helping St. Louis defeat the Boston Bruins in a seven-game Stanley Cup Final for its first championship since entering the NHL in 1967. He became the first non-goalie in NHL history to win the Conn Smythe in his first season after being traded. He had 23 points (eight goals, 15 assists) in 26 playoff games.

O'Reilly was chosen by Erie with the first selection of the 2007 Ontario Hockey League Priority Selection Draft and lived up to the billing as he was named Erie's Rookie of the Year after collecting 52 points in 2007-08. In 2008-09, he scored 66 points and was voted the best penalty killer in the OHL's Western Conference.

The Colorado Avalanche selected O'Reilly in the second round (No. 33) of the 2009 NHL Draft and O'Reilly became the first player to jump directly into the NHL in his draft year who wasn't a first-round selection since Patrice Bergeron of the Bruins and Dan Fritsche of the Columbus Blue Jackets in 2003-04.

O'Reilly's first NHL point was an assist in Colorado's season-opening victory against the San Jose Sharks on Oct. 1, 2009. At 18 years, 236 days, O'Reilly, at the time, became the youngest player in Avalanche history.

His first NHL goal was a game-winner against the Montreal Canadiens on Oct. 15, 2009, and he led the Avalanche in 2009-10 with 69 takeaways. He made his Stanley Cup Playoff debut April 14, 2010, against the Sharks, and his first playoff goal was an overtime winner 51 seconds into OT in Game 3 on April 18, 2010.

O'Reilly led Colorado in takeaways with 72 in 2010-11, and in 2011-12, he was Colorado's leading scorer with 55 points and topped the NHL with 101 takeaways.

He finished tied for fifth on the Avalanche in points with 20 in 2012-13. The following season, O'Reilly scored a team-high 28 goals, led the NHL with 83 takeaways and won the Lady Byng Trophy for skillful and gentlemanly play. He took one minor penalty in 80 games to become the second NHL player after Butch Goring of the 1977-78 Los Angeles Kings to receive two or fewer penalty minutes while playing at least 80 games.

O'Reilly appeared in all 82 games for the Avalanche in 2014-15, scoring 55 points and tying for the League lead with 98 takeaways.

Traded to the Sabres on June 26, 2015, O'Reilly led the team with 60 points. On March 31, 2016, O'Reilly assisted on a goal by his brother Cal during a 4-1 win against the Toronto Maple Leafs.

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