Colorado Avalanche (2009–2015)

After the 2008–09 OHL season, O'Reilly was drafted in the second round, 33rd overall, in the 2009 NHL Entry Draft by the Colorado Avalanche.^[5] He became the youngest player in Avalanche history to appear in a regular season game when he surprisingly made the opening night roster for the 2009–10 season.^[6] He earned his first NHL point in his Avalanche debut on October 1, 2009, against the San Jose Sharks, becoming the first non-first-round draft pick to immediately make the NHL in his first post-draft year since Patrice Bergeron in 2003–04.^[7] O'Reilly later scored his first career NHL goal, a game-winner, on October 15 against Carey Price of the Montreal Canadiens.^[8] On October 19, following his strong start to the season, the Avalanche announced that O'Reilly and his fellow 2009 draftee Matt Duchene would spend the entire season with the Avalanche.^[9] Initially leading all rookie forwards in scoring through October, O'Reilly recorded an assist in a 3–1 victory over the Detroit Red Wings on October 24 to cap a seven-game point streak, which stood as the longest by a rookie throughout the season.^[10]

Suffering an offensive plateau midway through the season, O'Reilly anchored his position as a top defensive forward and penalty killer. He led the Avalanche in ice time on the penalty kill, culminating in posting two short-handed goals during a double minor penalty against the Columbus Blue Jackets on February 2, 2010, to become the first Avalanche player in history to achieve two shorthanded goals in one game.^[11] He finished his rookie season with the Avalanche with 26 points (eight goals and 18 assists) in 81 regular-season games before making his Stanley Cup playoff debut in a 2–1 victory over San Jose in Game 1 of the Western Conference Quarterfinals on April 14. On April 18, he scored his first career playoff goal, scoring the game-winning tally in Colorado's Game 3 1–0 overtime victory over San Jose. He was credited with the goal after inadvertently deflecting Sharks defenceman Dan Boyle's pass attempt from an improbable angle into the net past goaltender Evgeni Nabokov to put the Avalanche ahead in the series, two games to one.^[12]

In his second professional season in 2010–11, O'Reilly remained entrenched on the shut-down third line for the Avalanche. On November 19, 2010, he appeared in his 100th NHL game, a 5–1 victory over the New York Rangers.^[13] In scoring eight goals in his last 18 games, O'Reilly was able to match his rookie season contribution of 26 points and led the team for a second consecutive season in takeaways, with 72.

In the following year, 2011–12, having established himself as the leading defensive centre on the Avalanche, O'Reilly developed his two-way game and increased his offensive production playing alongside linemate Gabriel Landeskog. This was recognized as he was named as the NHL's Second Star of the Week for November 28 to December 4 after contributing with seven points in four games, and was leading all Avalanche forwards in ice time.^[14] On January 12, 2012, he appeared in his 200th career NHL game, scoring a goal in a 3–2 overtime defeat to the Nashville Predators.^[15] Despite the Avalanche missing the playoffs for a second-straight season, O'Reilly lead the NHL and set an Avalanche record with 101 takeaways. He also more than doubled his points output for his previous two seasons combined to finish as top scorer on the Avalanche with 18 goals and 55 points.^[16]

As a restricted free agent in the off-season, O'Reilly's protracted contract negotiations with the Avalanche were halted by the expired Collective Bargaining Agreement between the NHL and the NHL Players' Association (NHLPA), which incurred the 2012–13 NHL lock-out.^[17] With the lock-out cancelling the start of the season, O'Reilly, in joining his brother Cal, signed a two-year contract with Russian Kontinental Hockey League (KHL) club Metallurg Magnitogorsk on December 7, 2012.^[18] Unlike the exodus of NHL players joining European leagues on short-term contracts, O'Reilly signed a two-year contract with the conditional opt-out clause in returning to the NHL if an agreement with the Avalanche was met. O'Reilly made his debut for Magnitogorsk in a 5–3 victory over Atlant Moscow Oblast on December 19.^[19]

Despite an end to NHL lock-out announced on January 12, 2013, O'Reilly was still unable to come to terms with the Avalanche and initially continued to play in Russia to miss the opening of the shortened 2012–13 season. On January 24, citing an ankle injury suffered through his 12 games with Metallurg, he mutually terminated his contract in Russia and returned for rehabilitation to North America.^[20] At an impasse with the Avalanche a month into the season, O'Reilly was training with the OHL's London Knights when he ended his holdout on February 28 after signing a two-year, $10 million offer sheet with the Calgary Flames.^[21] With a game coinciding between the two clubs on that day, the Avalanche announced during the contest that they opted to forgo compensation and secured O'Reilly to a contract by matching the Flames' offer sheet.^[22] Due to O'Reilly having signed a contract with a KHL franchise, had Colorado not matched Calgary's offer sheet, he would have had to pass through waivers before returning to the NHL.

In the 2013–14 season, under new Head Coach Patrick Roy, O'Reilly was moved to the wing to play alongside centre Matt Duchene and subsequently responded with a team-leading—and career-high—28 goals and 64 points in 80 games as the Avalanche returned to the playoffs. For the second time in his career, O'Reilly led the NHL in takeaways with 83 and, in recording just one penalty minor, was awarded the Lady Byng Memorial Trophy for gentlemanly conduct and sportsmanship by the League.^[23]

As an impending restricted free agent, and with past difficulties in contract negotiation, the Avalanche elected to take O'Reilly to a salary arbitration.^[24] On July 23, 2014, O'Reilly and the Avalanche agreed to settle prior to the arbitration hearing after negotiating a two-year, $12 million contract.^[25] During the 2014–15 season, O'Reilly scored 17 goals and 38 assists for 55 points, also leading the Avalanche in average ice time per game, as Colorado failed to make the 2015 playoffs.^[26]

Buffalo Sabres (2015–2018)

On June 26, 2015, the Avalanche traded O'Reilly, alongside Jamie McGinn, to the Buffalo Sabres in exchange for Nikita Zadorov, forward Mikhail Grigorenko, prospect J. T. Compher and the 31st pick in the 2015 NHL Entry Draft.^[27] On July 2, O'Reilly then signed a seven-year contract extension with Buffalo worth $52.5 million, which came into effect during the 2016–17 season. O'Reilly's signing came on the same day as the Sabres organization signed his brother Cal O'Reilly to a two-year contract.^[28] O'Reilly was immediately named an alternate captain upon his arrival in Buffalo. The Sabres improved in O'Reilly's first season, after finishing last in league standings the previous two seasons. O'Reilly led the team offensively with 60 points.

On April 7, 2018, O'Reilly surpassed Rod Brind'Amour for most faceoff wins in a single season (1,269) in a 4–3 loss to the Florida Panthers.^[29] At the end of the 2017–18 season, O'Reilly was named a Lady Byng Memorial Trophy finalist as the league's most gentlemanly player.^[30] In spite of his personal success, the Sabres failed to qualify for the NHL playoffs during his tenure, resulting in O'Reilly admitting he began to lose his love of the game.^[31] His comments seemed to polarize fans in Buffalo, with many subsequently labeling O'Reilly a cancerous presence on the team, while others defended his remarks and agreed that the Sabres had become complacent with losing.

St. Louis Blues (2018–2023)

On July 1, 2018, O'Reilly was traded to the St. Louis Blues in exchange for Tage Thompson, Vladimír Sobotka, Patrik Berglund, a 2019 first-round pick and a 2021 second-round pick.^[32] Although O'Reilly experienced a career-high season with the Blues, the team fell to last place around the entire league by January 2, resulting in a coaching and goaltender change.^[33] As the Blues rebounded, O'Reilly enjoyed a career year in his first year in St. Louis, achieving career highs in assists and points and tying his career high in goals.^[34] He was invited to his second career NHL All-Star Game, where he registered seven points through two games and was a finalist for the All-Star Game MVP, ultimately losing the award to Sidney Crosby.^[35]

As the Blues qualified for the 2019 Stanley Cup playoffs, O'Reilly became the first player to score a goal in four consecutive Stanley Cup Finals games since Wayne Gretzky in 1985.^[36] He was awarded the Conn Smythe Trophy after recording a franchise-record 23 playoff points^[37] as the Blues won their first Stanley Cup in franchise history, defeating the Boston Bruins 4–1 in game 7 of the 2019 Stanley Cup Finals.^[38] For his excellent two-way play during the season, O'Reilly won the Frank J. Selke Trophy as the league's best defensive forward on June 19, 2019, over Patrice Bergeron of the Boston Bruins and Mark Stone of the Vegas Golden Knights.^[39]

Although the 2019–20 season was cut short due to the COVID-19 pandemic, O'Reilly continued his success with the Blues and recorded 61 points in 71 games.^[40] Due to the loss of Vladimir Tarasenko to injury, O'Reilly was appointed an alternate captain for the team on October 31.^[41] On January 11, 2020, O'Reilly, as well as fellow Blues teammates Alex Pietrangelo, Jordan Binnington, and David Perron, were invited to the 2020 National Hockey League All-Star Game.^[42]

O'Reilly was named captain of the St. Louis Blues as replacement of Alex Pietrangelo on December 23, 2020.^[43]

Toronto Maple Leafs (2023)

With the Blues out of the playoffs picture during the 2022–23 season and O'Reilly an impending unrestricted free-agent, rumours began to circulate regarding a potential trade out of St. Louis for the team to recoup some assets. On February 17, 2023, the Blues traded O'Reilly and Noel Acciari to the Toronto Maple Leafs, in a three-team trade, also involving the Minnesota Wild. The trade sent the Maple Leafs' first-round pick and the Ottawa Senators' third-round pick in the 2023 NHL Entry Draft, the Maple Leafs' second-round pick in 2024, and Adam Gaudette and prospect Mikhail Abramov to the Blues. Toronto also received prospect Josh Pillar from the Wild, who got the Maple Leafs' fourth-round pick in 2025 in return, while St. Louis retained 50 percent and Minnesota retained 25 percent of O'Reilly's salary as part of the trade.^[44][45] He got his first Leafs point that game and his first Leafs hat trick on February 21 against his former team, the Buffalo Sabres. ^[46]

After recording five points in his first three games with the Maple Leafs, O'Reilly went five games scoreless before suffering a broken finger in his eighth game with the team while blocking a shot; his subsequent surgery to repair the finger. The injury made him miss 4 weeks. In his game back he picked up 3 assists against the Montreal Canadiens.^[47]

Nashville Predators (2023–present)

In leaving the Maple Leafs, O’Reilly signed with the Nashville Predators on July 1, 2023, the opening day of NHL free agency. In joining his fifth NHL team, he was signed to a four-year, $18 million contract to begin in the 2023–24 season.^[48]