"He gives us a lot of roster flexibility," president of baseball operations Andy MacPhail said of Fox, who can play the outfield, corner infield and catcher positions.
"One of the appeals is a potent right-handed bat, potentially off the bench if he's not playing."
Fox was designated for assignment on June 12, meaning the A's had 10 days to trade him, grant him his outright release or hope he cleared waivers. Given that Baltimore (19-50) expressed some interest, it became clear that the latter option was unlikely.
The 28-year-old Fox was batting .214 with a .264 on-base percentage through 106 plate appearances when the A's designated him, but if he gains his stroke, he could bat his way into the lineup on an everyday basis.
The Orioles entered Tuesday's game with an American League-low 225 runs scored, and MacPhail was hopeful Fox could revert back to last season's form, in which Fox hit 11 homers with 44 RBIs in 82 games with the Cubs.
"I think it's just the consistency in playing time," Fox said of his drop in numbers. "For me, I'm the type of player -- especially with a lot of power hitters, it's all about timing, it's all about consistency at the plate. And when you get infrequent at-bats -- I was playing once or twice a week [in Oakland] -- it's very difficult to perform the way you want to perform.
"Especially after what's happened, I'm going to do what it takes to help the team win. Obviously I'm pretty versatile. I'm here to help any way I can, and if that earns me more playing time, then I'll be really excited about that."
Interim manager Juan Samuel said Fox will be used as a utility guy, predominantly at left field and first base, but he could spell relief for catcher Matt Wieters or come off the bench.
"We're going to do anything we can to get him in there and get some offense," Samuel said, adding that Fox's presence could mean increased time at designated hitter for Wieters.
"[Fox is] just an extra guy to help us off the bench, to pinch-hit, if we need to throw him in against a left-hander."
The move further complicates things for underperforming Garrett Atkins' right-handed bat, which has been relegated to the bench. When asked what Fox's presence does in terms of finding Atkins at-bats, Samuel joked, "Add a little headache."
Atkins has made just two starts in the past two weeks and has one homer and nine RBIs in 137 at-bats this season. He enters Tuesday batting .219, and his roster spot could be on shaky ground in the coming days.
Samuel said he spoke to Atkins in San Diego to let him know he hadn't forgotten about him, and said Atkins has handled the situation like "a professional."
"Now that we're back in the American League, we have our DH back and we can get playing time for those guys there," Samuel said.
To make room for Fox on the roster, Baltimore optioned right-hander Chris Tillman to Triple-A Norfolk and designated Norfolk reliever Cla Meredith for assignment. Tillman was 0-3 with an 8.40 ERA in four starts for the Orioles, while Meredith was 0-2 with a 5.40 ERA in 21 games. After he was optioned to Norfolk, Meredith went 1-0 with a 13.50 ERA in five games for the Tides.
Brittany Ghiroli is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.