Orr will be a backup infielder, who will get occasional starts. Entering Saturday's action, he was hitting .267 with two home runs and 30 RBIs for Columbus.
Orr was one of the last cuts in Spring Training after hitting .395 with a homer and four RBIs.
Orr has spent most of his career with the Braves and helped them reach the playoffs in 2005.
Shell, 25, went 3-2 with one save and a 2.62 ERA in 22 games (four starts) for Columbus. He also recorded 54 strikeouts in 58.1 innings. He went 2-1 with one save and a 1.73 ERA in 18 relief outings, compared to 1-1 with a 4.09 ERA in four starts. Shell's first appearance with the Nationals will mark his Major League debut.
Mock, 25, made two starts with the Nationals, going 0-2 with a 6.97 ERA. Sanches, 29, was 2-0 with a 7.36 ERA in 12 relief appearances with Washington.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.