O'Reilly's deal is worth $52.5 million, reports Elliotte Friedman of Sportsnet, including $1 million per year in salary and the rest in signing bonuses.
The Sabres acquired O'Reilly in a multi-player trade late last month with the Colorado Avalanche. He spent the previous six seasons with the Avalanche after being selected in the second round of the 2009 draft.
Over 82 games in 2014–15, O'Reilly scored 17 goals and recorded 38 assists. He has registered at least 55 points in three of the last four seasons.
Last season the Sabres went 23–51–8 and finished in last place in the Eastern Conference.