Oswalt, who pitched for most of the game with a torn nail on his right middle finger, had thrown 99 pitches through eight innings and appeared to be headed toward his second complete game of the year. Those aspirations disappeared, however, when Daryle Ward knocked a pinch-hit single with one out, and Alfonso Soriano followed with a base hit to almost the exact same area.
Manager Cecil Cooper called for Jose Valverde, owner of a franchise-record 10 saves in a row.
"It was not [nerve-racking] for me," Cooper said. "I'm sure for a lot of people it was, but I have complete faith in those guys, particularly Roy, and Valverde at the end of the ballgame. We do what we can to get [Oswalt] as deep as we can, and then we hand it over to the big boy."
The big boy -- in this case, Valverde -- put the Wrigley Field crowd of 40,670 in a temporary frenzy when one of the Cubs' most feared hitters, Derrek Lee, sent a long fly ball toward right field. Hunter Pence twisted and turned and dashed toward the warning track, where he caught the ball for the last out.
"When he hit it, I had an idea of where it was, just [by] the angle of the way it was carrying," Pence said. "It was going to be a funky turn for me, either way, whether I went over my right or left shoulder. I tried to get to my spot and tried to put myself in a position to catch the ball."
The game-ender capped Oswalt's 13th win, in his longest outing of the year. The right-hander yielded four hits, walked none and struck out three.
"Just a terrific outing by Roy -- magnificent, if you want to choose a word," Cooper said. "Just magnificent. Just an overpowering performance."
Oswalt tore the nail on his finger early in the game and said he felt it while throwing his breaking pitches more than anything else. Cooper and the athletic training staff monitored the issue throughout the game, but Oswalt assured them he was fine.
Prior to the ninth, Oswalt held the Cubs to two hits. He yielded a leadoff single to Geovany Soto in the third frame and didn't allow another until Ryan Theriot singled to second to lead off the sixth.
"My slider felt pretty well," Oswalt said. "The fastball was coming out of my hand real well. That's the biggest key -- I was able to spot the ball down in the zone. It gets you quick outs as far as ground balls in the infield. I'm not getting behind anybody."
"He's good," Lee said. "He's always good. We probably made him look a little better than he really was -- one hit or two hits or whatever he gave up, we should be able to do better than that -- but Oswalt is always good."
Jason Marquis was, too. He kept the Astros relatively quiet until the fourth, when Miguel Tejada, leading off the inning, sent a line drive off the ivy wall in left. The ball bounced away from Soriano, who sprinted after it but could not nab it before it rolled over the pitcher's mound in the bullpen and kept going.
"It reminded me of when I was a little kid trying to catch a chicken," Soriano said. "We did that in the Dominican."
Soriano fumbled with the ball during his first attempt to pick it up, giving Tejada time to reach third.
"I was thinking two," Tejada said. "When I hit the ball, I just started running hard to get two. I put my head up when I got to second base, and I kept running. I just took my chances."
While watching Tejada run around the bases, Cooper was in the dugout screaming, "Keep going, keep going."
"I felt a whole lot better when he got to third," Cooper added.
Tejada's aggressiveness was soon rewarded. Lance Berkman drew a walk, and Tejada scored on Geoff Blum's sacrifice fly. Berkman moved to second on the play at the plate, stole third and scored on Pence's base hit up the middle.
"We had some real good baserunning today, particularly Lance," Cooper said. "He stole one for us there and got the second run."
The Astros have won six games in a row and are six games over .500. They're still a long shot for the Wild Card, 8 1/2 back in that race. They're 13 behind the Cubs in the National League Central division race.
Still, Cooper is pleased with his club's recent results.
"We have played really good baseball the second half," Cooper said. "The guys have all picked it up, everybody's contributed. If I had to pick one thing, I would say pitching's been really, really good for us.
"It's fun to be competing against the Cubs and the Brewers and Cardinals, in your division, where every day is kind of battle for you, go out every day and compete against those guys. I think this is the best division in baseball."
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.