If Buffalo really wants Carlson, and is able to sign him to a long term deal that Washington isn't ready to match, I think Washington still wants to contend this year, so they either want Scandella or O'Reilly back.
https://www.hockey-reference.com/players/s/scandma01.html
https://www.hockey-reference.com/players/c/carlsjo01.html
Scandella is only 50 days younger, plays two fewer minutes a night this year, three fewer, career, has a third of Carlson's offense this year (and a tenth of his goals), and just under a third of his offense, for his career.
But as far as defensemen on teams that aren't going to make the playoffs this year go, Scandella is one of few guys who at least dimly compares to Carlson's minutes. If the Caps give up Carlson for Scandella, they want something thrown in that allows them to add at least a 20 minute right handed defenseman for the playoffs. Obviously OEL would be a better fit if they're trading Carlson for a left defenseman and then joining the Green sweepstakes prepared to give up a draft worth of picks.
Carlson for ROR straight up, though, would probably be fine. O'Reilly is on pace for his second season of about thrity goals (he hit twenty-eight about five years back?), and his strength on defense and on faceoffs make him a very complete 200 foot player whom Trotz and MacLellan would covet. Carlson, of course, is fourth in scoring among defensemen and playing an unreal 25 minutes a night with a corsi that's 2% better than his offensive zone starts, so he's also a solid two hundred foot player, but he hasn't re-upped his contract yet, so maybe another team is willing to match his demands---kind of like the situation that brought O'Reilly to Buffalo.
https://www.hockey-reference.com/players/o/oreilry01.html
Actually, if the Caps aren't planning on keeping Carlson, I think they can throw in a couple of forward prospects and some kind of a draft pick to land O'Reilly, who would give them the depth at center to really contend this year and outmuscle teams like Pittsburgh and Tampa, even if losing Carlson leaves them with a gaping hole at right defense that they won't have cap space to fill through free agency, and they will have to seriously overpay to get a veteran with retained salary to steady the ship.
Buffalo, once they've got Carlson can build around him and hope he keeps improving and wins a Norris Trophy in the next couple of years, or they can flip him, the same day, for picks and prospects, and keep building through the draft around Eichel, with a lot more salary cap flexibility.