Pretty much the only players I wouldn't give up for O'Reilly are Hodgson and Grigorenko. He'd be an absolutely amazing get for the Sabres, it's ridiculous that the Avs let it get to this point. He's a guy that is renowned for his work ethic, one of the best two way forwards in the entire league, and he turned 22 less than a week ago. It feels like he's older because he's been in the NHL since he was 18, but trading a guy like Ennis for him would actually make the Sabres younger.
It's about money and not playing time, so it's not identical to the Hodgson situation, but it's similar in the fact that a really young, talented center that should never be available somehow is available. It'd cost a pretty penny, like it did for Hodgson, but getting O'Reilly on this team (and assuming if they'd trade for him they'd agree to a contract pretty quickly) makes Buffalo better short and long term. I really hope Darcy is tirelessly working the phones on this one.
edit: If they're looking for a "roster player and top prospect", Colorado would probably ask the Sabres about Ennis, Hodgson, Foligno, Sekera as the roster player and Armia, Girgensons, or maybe McNabb/Pysyk as the prospect. We can stop with the Stafford proposals too... literally every team in the NHL would make room for O'Reilly. Most teams don't have the cap space or prospects to pull an O'Reilly trade off (and then meet his contract demands), but Colorado isn't accepting some low-ball offer centered around Drew Stafford for him. Seriously doubt the Avs would take on Stafford's $4M/yr with the way they've penny pinched during the O'Reilly negotiations, especially with a very similar streaky scoring winger (who also signed a 4/16 contract recently) already on the team in David Jones. Plain and simple, Buffalo has absolutely no chance of landing him if the best player they're offering in return is Stafford.