Oswalt (9-13) has won three straight starts for the first time this season, helping the Phillies try to make a move on Atlanta for the NL East lead.
Oswalt allowed five hits, and the Phillies backed him with the kind of run support he rarely got in Houston before getting traded in July. He never came out for the eighth after the game was delayed 1 hour, 44 minutes by rain.
The right-hander also got two hits and scored a run. Oswalt pitched out of early-inning jams, leaving a runner stranded at third base in the first and fourth innings. Oswalt struck out the final two batters of each inning to end those threats.
Oswalt is 3-0 and has allowed only five earned runs over his last four starts. He'd received the second-lowest run support (2.61 runs) of any starters in the majors this season.
Ryan Madson threw a scoreless eighth and J.C. Romero worked the ninth for Philadelphia.
The Phillies bounced back a day after they were beaten 8-1 by Stephen Strasburg and the Nationals bullpen.
Philadelphia is 22-7 since July 22 and has a fourth straight NL East title in its sight. Ryan Howard and Chase Utley, who both returned from the disabled list in the last week, sat out Sunday, presumably healthy and fresh for the Phillies' ongoing run of 24 games in 23 days.
Raul Ibanez hit his 12th homer of the season, a two-run shot, not long after the tarp was removed in the seventh for a 6-0 lead. Placido Polanco had two RBIs on a sacrifice fly and single, and Wilson Valdez had a two-run single in the sixth. The Phillies scored four runs against Nationals starter Scott Olsen (3-6).
Oswalt's start means he'll miss pitching against his former Houston teammates during a four-game series that opens Monday.
The Nationals put a runner in scoring position with less than two outs in each of the first four innings. Oswalt fanned Adam Dunn and Ryan Zimmerman to end the first, and whiffed Michael Morse and Adam Kennedy to finish the fourth.
Jimmy Rollins hit a leadoff double in the Phillies first and later scored on Polanco's sacrifice fly. Polanco's RBI single in the third drove in Oswalt.
With runners on second and third and two outs in the sixth, Olsen intentionally walked Carlos Ruiz. Valdez, who hit into a double play in the second, lined a single to center for a 4-0 lead.
NOTES: The Nationals will wait for results of an MRI exam on Strasburg before deciding what to do with their ace right-hander. Strasburg strained a tendon in his right forearm in Saturday night's start. He was scheduled to have the MRI on Sunday. ... The Phillies honored umpire Jerry Crawford for his 33 years of service. Crawford will retire at the end of the season. ... Nationals analyst, and former Reds closer, Rob Dibble said he'll miss manager Lou Piniella. Piniella, who retired Sunday, managed Dibble and the Reds to the 1990 World Series championship. "As far as respecting the game, Lou taught me an awful lot about that," Dibble said.