Oswalt, who has been dealing with tightness in his lower back, set a club record by getting hung with his 16th no-decision in his 30th start of the season, breaking a tie with Pete Harnisch and Jim Deshaies. Oswalt hasn't received a decision in any of his four starts this year against the Reds, keeping his record against Cincinnati at 23-1.
"It's one of those years," Astros manager Cecil Cooper said.
Houston was playing without first baseman and Reds destroyer Lance Berkman, who was scratched from the lineup with back spasms. But another Rice University product -- Janish -- enjoyed a big night at the plate with three doubles and drove in the game-winning run in the seventh.
"I was kidding around with some of their guys," said Janish, a Houston native. "Some of our guys said this game is televised in Houston tonight so you have to do something. I know a lot of those guys over there, a lot of the coaches like [first-base coach Jose] Cruz. I played with [his son] Enrique [Cruz] in college. It's good to play in front of the home fans, even though you're not in the hometown. It's still on TV, so it's good."
Cooper brushed off Janish's big day.
"Nothing special," he said. "Am I supposed to say something special about him? Why? Because he's a Rice product? C'mon. He doesn't play for us, so I don't really care."
Houston had three hits and stole three bases in the first inning against Reds starter Kip Wells, and all three of the batters who stole a base -- Michael Bourn, Kazuo Matsui and Miguel Tejada -- scored a run. Matsui, Tejada and Geoff Blum had RBI singles to give the Astros a 3-0 lead.
"In the first inning, we were aggressive," said Matsui, who was a late addition to the lineup when Berkman was scratched. He went 2-for-4 and is hitting .325 in his past 21 games.
Oswalt gave up a homer to Drew Stubbs on the first pitch he threw and a double to Janish on the second pitch. Janish came around to score to cut the lead to 3-2, but Oswalt retired 14 of the next 15 batters he faced heading into the sixth inning.
"I was kind of timid in the first inning," Oswalt said. "In the back of your mind you don't want [the back] to lock up in the first. After the second, I got my mechanics down a little bit better."
"He pitched much better after the first," he said. "It looked like he started to use his offspeed stuff. His command was much better. In the first inning he just mislocated some pitches, and they hit them. He said after the fifth he kind of tightened up a little bit and he wanted to go out in the sixth and give it a whirl."
Matsui launched his sixth homer of the season down the right-field line in the third inning to extend the Astros' lead to 4-2. Oswalt was pulled after giving up consecutive doubles to Janish and Joey Votto to start the sixth. Janish scored on Brandon Phillips' grounder off Doug Brocail, and Scott Rolen followed with an RBI double to straightaway center to tie the game.
Gervacio hit Corky Miller with a pitch to start the seventh, and Janish ripped a double to right field two outs later to put the Reds ahead, 5-4.
The Astros managed only four baserunners after the first inning, and the only batter to reach second base during that span was Matsui when he homered in the third.
"Not much after that," Cooper said. "That's the key. We only had one solo home run and couple of singles with nobody on. We didn't do a whole lot after the first."