After a brief stint with the Colorado Avalanche, Eller joined the Pittsburgh Penguins in 2023, becoming the first Danish player to play in 1,000 NHL games, before being traded back to the Capitals in 2024.

Internationally, Eller has played for the Danish national team at both the junior and senior level, including at five World Championships.

His preseason was spoiled by mononucleosis and, as a result, he started the season playing for the Peoria Rivermen in the American Hockey League (AHL).

[4][5] On 17 June 2010, Eller was traded by the Blues to the Montreal Canadiens, along with Ian Schultz, in exchange for goaltender Jaroslav Halák.

[11] Eller was a healthy scratch for three games in mid-January before rejoining the lineup on 18 January to play alongside Plekanec and Andrei Kostitsyn.

[12] Upon re-joining the lineup, Eller surpassed his previous career-high of ice time by playing 16:53 minutes in a game against the Buffalo Sabres.

[13] In late February, Eller found chemistry alongside Travis Moen and Kostitsyn and the three carried a five-game, seven-point streak by 4 March.

[25] During the 2012–13 2012–13 NHL lockout, Eller signed a temporary contract with JYP Jyväskylä in the Finnish elite hockey league Liiga.

While with JYP, Eller played alongside Boston Bruins forward Rich Peverley and ex-NHLers Éric Perrin and Ramzi Abid.

[28] He also earned a spot of the penalty kill as a replacement for an ill Max Pacioretty[29] where he tallied a goal and two assists while going 63% in the faceoff circle during a 6–1 win over the Sabres in early February.

[30] Later that month, Eller was paired alongside Alex Galchenyuk and Erik Cole and they immediately made an impact on the team.

[32] During the 2013 Stanley Cup playoffs, Eller was knocked unconscious following an open ice check by Ottawa Senators' defenceman Eric Gryba.

[38] While on this line, Eller tallied nine points in 13 games during the month of October and led the team in goals during the early stages of the season.

[1] As the Canadiens qualified for the 2014 Stanley Cup playoffs, Eller continued to produce and set career-highs in games played, goals, assists, and points.

[1] Following the signing of his contract extension, Eller spent six weeks in his hometown of Copenhagen before returning to Montreal to resume his daily workouts at the Bell Sports Complex.

[46] After six seasons, Eller's tenure with the Canadiens came to an end at the 2016 NHL entry draft, as he was traded to the Washington Capitals in exchange for two second-round picks in 2017 and 2018 on 24 June 2016.

[47] Upon joining the Capitals for the 2016–17 season, Eller centred their third line alongside André Burakovsky and Tom Wilson for their opening night matchup.

[56] As the Capitals began trailing the Penguins, Eller gained new line mates in captain Alexander Ovechkin and Tom Wilson during Game 6.

[58] Following their early elimination, the Capitals released their protection list prior to the 2017 NHL Expansion Draft which included Eller.

[67] As a result of numerous injuries to the Capitals in November, Eller earned more playing time while centering the top line alongside Ovechkin and Tom Wilson.

[81] On 1 March 2023, the Capitals traded Eller to the Colorado Avalanche in exchange for a second-round pick in the 2025 NHL entry draft.

[82] With the Capitals in the midst of a West Coast road trip, Eller immediately made his debut with the Avalanche that night at Ball Arena in Denver, Colorado, playing in a third-line center role of a 7-5 defeat to the New Jersey Devils.

His father, Olaf Eller, is a former Danish international ice hockey player and former coach of several teams in the Superisligaen, as well as Troja/Ljungby in Sweden.

Olaf Eller is also a color commentator for Danish TV 2 Sport for the IIHF World Championships and occasionally for games in the AL-Bank Ligaen.