Purcey, four days shy of his 29th birthday, was designated for assignment by Toronto last week after struggling with command out of the bullpen, tallying four walks in just 2 1/3 innings.
He made five total appearances, including two against the A's in the first week of the season. Against Oakland, he fanned three in a combined 1 1/3 scoreless innings. Purcey had yet to allow a run this year until his most recent outing against Seattle, when he surrendered three in just one-third of an inning.
"He had one bad outing the other night, but he pitched well against us, and we feel like he has a chance to contribute in the 'pen," A's assistant general manager David Forst said.
The left-hander, who is out of options, takes the active roster spot of Dallas Braden, who was placed on the disabled list with a stiff left shoulder on Monday. In turn, the A's transferred right-hander Rich Harden (shoulder) to the 60-day DL.
Purcey, whom the A's will need to keep on the 25-man roster or risk losing him on the waiver wire, came up through the Blue Jays' system as a starter, but was converted to a reliever last season. The transition proved successful, as he posted a 3.71 ERA with 32 strikeouts and 15 walks in 34 frames.
"He had a really good year in his first season as a full-time reliever," Forst said. "He's always had a good arm. His velocity was good this spring and this season."
The southpaw owns an overall 5-10 with a 5.42 ERA in 59 career games with Toronto, and he boasts a fastball that often finds the 94-95-mph zone. The A's called for his services as soon as he was made available by the Blue Jays, Forst noted.
Purcey's presence means the A's now have five left-handed options in the bullpen. Right-handers Grant Balfour and Brad Ziegler are awaiting the return of fellow righty Michael Wuertz, who is rehabbing a strained hamstring in the Minors. Upon Wuertz's reinstatement from the DL, which currently has no timetable, the A's are then likely to option a southpaw.
Farquhar, 24, heads back across the border just five months after Oakland acquired him alongside pitcher Trystan Magnuson for outfielder Rajai Davis. The righty had thrown eight scoreless innings with nine strikeouts in four relief appearances for Triple-A Sacramento this year.
The A's have been involved in 13 trades with the Blue Jays since Billy Beane assumed duties as Oakland's general manager following the 1997 season.