The A's were trailing 3-0 before launching a rally in which they scored the game's final four runs.
Then in the eighth, Nate Freiman collected a lead-off single against Hector Ambriz. One out later, Wesley Wright (0-4) came in to face Reddick and the A's outfielder blasted a pitch into the right field seats for the decisive blow.
Ryan Cook (3-2) earned the decision and Grant Balfour racked up his 26th save for American League West-leading Oakland reeled off its 10th straight win against Houston.
Chris Carter and J.D. Martinez each drove in a run for the Astros, who dropped a sixth straight overall.