Richard (4-7) turned in his second straight strong start after handling the Seattle Mariners on one run over seven innings Tuesday.
The left-hander walked two and struck out six while locating 79 of his 115 pitches for strikes as the Padres salvaged one win in their three-game series in Oakland.
Huston Street delivered a four-out save, his eighth of the year, and Will Venable got the only RBI for San Diego.
Oakland's Bartolo Colon (6-7) allowed one run on one hit before exiting after two innings with a strained oblique muscle.
Coco Crisp had two hits, including an RBI single, for the A's.