McFarland, who has appeared in 23 games this season and started one, is 4-2 with a 3.07 ERA, 23 strikeouts and 11 walks this season. The lefty has served as the O's primary long reliever this season, giving the Orioles 10 multi-inning relief appearances.
Since the start of June, McFarland has a 2.10 ERA, 16 strikeouts and six walks.
Guilmet didn't pitch during his latest stint in the Majors -- his fifth time on the 25-man roster this season.
"I love being the guy that they know they can rely on me," Guilmet said. "I just take the opportunities when I get them and make the best of them."
McFarland's return to the active roster may not last long. Baltimore added relief pitcher Andrew Miller in a trade with the Red Sox on Thursday and he is expected to report by Friday's series opener against the Mariners. At that point, the O's will option a relief pitcher.
Brad Brach and Ryan Webb are also candidates to be optioned.
David Wilson is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.