The two-time NL champions are 15-4 since July 22, but lost another player when Ross Gload left with a strained right groin.
Oswalt (7-13) allowed five hits, walked two and struck out five. Relying on a powerful fastball and sharp curve, he mowed down the Dodgers to earn his first win in his third start for the Phillies.
The three-time All-Star right-hander, acquired from Houston before the non-waiver trade deadline, pitched twice on the road before putting on the red pinstripes for the first time.
Dodgers starter Clay Billingsley (9-7) gave up two runs and five hits in six innings.
After Oswalt blew a 95 mph fastball past pinch-hitter Matt Kemp on his 109th and final pitch, manager Charlie Manuel turned to his bullpen.
Ryan Madson worked around a leadoff double in the eighth, striking out two batters. Brad Lidge pitched a perfect ninth for his 15th save in 19 chances.
The injury-depleted Phillies already were missing All-Stars Ryan Howard and Chase Utley and Shane Victorino before Gload got hurt running out a double in the sixth. The backup first baseman will be re-evaluated on Thursday.
Oswalt has received the lowest run support of any starter in the majors this year. It was 2.48 runs per game coming in. He didn't need much this time.
The Phillies took a 1-0 lead in the fourth when Billingsley struggled with his control. Placido Polanco led off with a single. Gload followed with a sinking liner to left, but Jay Gibbons made a diving catch to rob him.
Billingsley then walked Raul Ibanez and Jayson Werth to load the bases. Polanco scored on Domonic Brown's grounder.
Ibanez made it 2-0 with an RBI double in the sixth. Gload led off with a double, limping to the bag after getting hurt. Ibanez hit a drive to the gap in left-center to knock in pinch-runner Mike Sweeney.
Philadelphia wasted a leadoff triple by Wilson Valdez in the fifth. Oswalt and Jimmy Rollins grounded out to drawn-in infielders and Polanco flied out.
NOTES: Phillies All-Star 2B Chase Utley has been cleared to hit, six weeks after surgery on his right thumb. Assistant GM Scott Proefrock said the team plans to stick with its original timetable of eight weeks for Utley's return, though it's possible he could be back sooner. ... Ibanez has an 18-game hitting streak. Polanco's reached 10 games. ... A crowd of 45,144 was the 96th straight sellout at Citizens Bank Park. ... Dodgers starters have a 2.63 ERA since July 17. ... Rollins got his 25th straight stolen base, dating to July 22, 2009. It's the longest streak in the majors. ... Victorino was 3 for 4 with a triple and homer in a rehab start at Triple-A Lehigh Valley. He's eligible to return on Thursday.