Per the team's press release:
The Washington Nationals announced that infielder Kory Casto and left-handed pitcher Mike O’Connor have cleared waivers and been outrighted to
Nationals Assistant General Manager and Vice President of Baseball Operations Mike Rizzo made the announcements.
This pretty much ensures that Wil Nieves and Josh "The" Bard will both be on the opening day roster.
It'll be a short stay for one of them though, as they'll have to make room for Jordan Zimmermann on April 19.