Oswalt (13-9) gave up four hits in 8 1/3 innings while winning for the fifth time in his past six starts.

Miguel Tejada was 2-for-4 with a triple and Lance Berkman had two hits and two walks for the Astros, who have won six straight games.

Jason Marquis (9-8) suffered the loss after giving up just two runs on five hits and three walks in six innings. He also struck out eight for the Cubs, who remained 4 1/2 games ahead of Milwaukee atop the NL Central, after the Brewers lost to the Mets on Monday.

Chicago had four hits, all singles, and lost its third in a row.