Hammel (7-2) didn't allow a hit through the first 6 2/3 innings, when Jason Heyward lined a solid single to right. But that was all Atlanta could muster off the right-hander, who broke a personal three-game winless streak.
Hammel yielded two walks as the Orioles evened their three-game series at Turner Field.
Atlanta starter Brandon Beachy (5-5) lasted only 3 2/3 innings before exiting the game with a sore elbow. He struck out five but walked Chris Davis, who later scored on Reynolds' RBI hit off reliever Anthony Varvaro.