Ryan O'Reilly's 27 and his career high in a season is 64 points and that was when he played wing on the Avalanche. He's usually good for around 50-60 points over a season which is solid production if you ignore his cap hit. But you can't because for 7.5M per, that's just not good enough.
Moreover, trading one of the top four defenceman for him would create a giant hole on the back end for what essentially culminates as a half measure. It's one thing to move Hamilton or Brodie for someone like Marner or Seguin because at least then you're getting a bonafide game breaker who can take over games offensively. But Ryan O'Reilly? Oh joy, another defensive specialist who can chip offensively at a slightly better pace than Backlund. That will fix everything.
Adding another layer, there's simply no way Treliving and co. bring in a player who makes more than Giordano and Gaudreau and is worse than both of them. There goes their entire cap management strategy not to mention potential lockerroom trouble (and no one say it's too unprofessional to happen; see Regehr). So the Sabres would have to retain and that would only increase the price making the deal even worse.
And to top off this terrible cake, it's highly likely the rebuilding Sabres would want a much younger asset under team control for far longer than 27 (soon to be 28) year old Brodie and the two years left on his contract as the principal piece.
It's not that O'Reilly wouldn't make the team better, it's just that it really makes no sense given the variables. The Flames can get similar half measures in free agency for nothing more than cap space. No need to decimate the defence for that.