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.
David Wilson is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.