Reynolds, who has not played since May 10 because of a strained left oblique, was in Monday's lineup as the designated hitter, batting eighth.
"Everybody was glad to see him. I know he's glad to be back," manager Buck Showalter said. "He's ready to go."
In two rehab games at Double-A Bowie, Reynolds went 1-for-7 with two walks and four strikeouts. Showalter plans to use him at multiple positions, including first and third, but will make that determination based on matchups.
Reynolds, now in his second season with the Orioles, struggled over 27 games before landing on the DL, hitting just .191 with a .661 OPS and two homers. The 28-year-old hit 30-plus homers in each of the past three seasons.
Though Showalter is glad to have Reynolds back, he is disappointed that Pomeranz's MRI revealed a tear.
"Not very encouraging," he said. "He has a pretty good tear in there. I don't think that will be an imminent DL return, that's the early read on it.
"It's a kick in the pants losing Pomeranz. He was a big help to our bullpen as a guy who could pitch multiple innings."
Pomeranz, who made his Major League debut on May 7, has made three appearances and has a 3.00 ERA. The 27-year-old logged multiple innings in two of his three outings, including Sunday's loss to the Royals, in which he sustained the injury.