Cincinnati went 14-12 in July following a 6-4 homestand.
Alexis Díaz (3-1) won despite allowing a tying home run in the eighth to Anthony Santander.
Buck Farmer got his first career save in six chances after an eventful ninth inning.
Rougned Odor appeared to reach leading off on a bunt when first base umpire Larry Vanover ruled Odor beat the throw from first baseman Ryan Mountcastle to Farmer covering the base. While the Reds were out of challenges, a crew chief video review reversed the call.
Terrin Vavra, pinch hitting in the pitcher’s spot after designated hitter Adley Rutschman replaced Robinson Chirinos at catcher in the seventh inning, walked with two outs. Trey Mancini followed with a game-ending groundout.
Santander had three hits, including his 19th homer, and extended his hitting streak to 11 games.
Reds rookie Nick Lodolo allowed one run and four hits in six innings with seven strikeouts.
Orioles starter Austin Voth gave up four hits in five scoreless innings with six strikeouts and no walks.
Cincinnati took a 2-0 lead in the sixth when Joey Votto beat the shift with an RBI single to left and Kyle Farmer hit a sacrifice fly.
Cedric Mullins hit a sacrifice fly in the seventh.
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