Fox hit .214 with two home runs and 12 RBIs in 39 games for the A's this season.
"Fox gives us another bat," Orioles interim manager Juan Samuel said. "He's also a catcher, plays first base, third base, outfield, so there's some flexibility for us."
Although the right-handed hitting Fox had poor numbers at the plate with Oakland, he gives Samuel another option in his effort to generate some punch on a team that ranks last in the AL in runs scored.
"We're going to do anything we can to get him in there and get some offense," Samuel said. "That's just an extra guy to help us off the bench, to pinch hit if we need to throw him in against a left-hander."
Fox hit .259 with 11 homers and 44 RBIs in 82 games last season. He hopes a change in scenery will revive his bat.
"I'm excited about it. It's good to go someplace where hopefully you fit in because where I came from, it just wasn't the right fit for anybody," he said. "I didn't fit into their style and it's very difficult to change your style at this point. You can't teach an old dog new tricks. So I'm happy to be here. It seems like they want me to be here."
Fox was traded from the Cubs to Oakland in December.
"When I came from Chicago ... Oakland had fielded the team they wanted," he said. "It was almost like they didn't want me, but they didn't want to get rid of me, either. So I was kind of stuck."
He was designated for assignment on June 14, which paved the way for his move to Baltimore.
Baltimore optioned right-hander Chris Tillman to Triple-A Norfolk to make room on the 25-man roster for Fox, and cleared a space on the 40-man roster by designating righty Cla Meredith for assignment. Tillman was 0-3 with an 8.40 ERA in four starts before being sent to the bullpen last week.
Tillman was not used out of the bullpen, and Samuel said allowing him to pitch every fifth day in the minors was essential for his development.
Meredith was 0-2 with a 5.40 ERA in 21 relief appearances with Baltimore and was ineffective in five games with Norfolk.