Stammen said location was the biggest reason for his strong performance.
"My fastball command was pretty good tonight," Stammen said. "You look at all the good [starts], that's probably what it is -- eliminating the amount of mistakes that I make. That's just pitching, and I'm learning as we go.
"I'm finally getting to the point where I need to take the next step as a starter."
Stammen has shown glimpses of his talent this season, but he has not been consistently good over his 18 starts. In fact, he was sent down to Triple-A Syracuse on June 6 to make room for rookie right-hander Stephen Strasburg.
But since coming back to Washington on June 26, Stammen has looked sharp in all but one of his five starts. In his past three outings, he has given up a total of five runs.
Nationals manager Jim Riggleman said it was nice to see his starter get back on track.
"He's had a couple days where he actually threw pretty good and we didn't really play that good or it happened to be a time where the bullpen didn't hold the lead," Riggleman said. "I think his numbers are skewed a little bit. It's a good time for him to come together. Hopefully he continues this."
Stammen also got something he hasn't had in a while: run support.
Thanks to Nyjer Morgan's early hitting, the Nationals scored the game's initial runs. Morgan led off the first inning by taking Oswalt's first pitch as a Phillie deep to center field for a triple. Adam Kennedy followed with an RBI groundout off Oswalt (6-13) to put the Nationals up, 1-0.
"I just basically wanted to get the party started off right, and it happened," Morgan said of his triple. "Same Oswalt. He's got electric stuff, but we just jumped on him."
Washington added to its lead in the third inning. With Stammen on second base and Morgan on first, Kennedy bunted down the third-base line. Philadelphia catcher Carlos Ruiz threw to third base, but no one was there. Stammen was able to score and Morgan went to third base. Ryan Zimmerman's sacrifice fly drove in another run to make it 3-0.
Josh Willingham helped the Nationals add to their lead in the fifth inning with a two-run double off Oswalt.
"I came out early a little amped up, holding the ball a little bit tight," Oswalt said. "[I] didn't really start feeling better until the last two innings actually. It didn't really go my way."
After Stammen allowed a homer to Jayson Werth, Washington added three more runs in the seventh off Chad Durbin on Roger Bernadina's two-run double and a sacrifice fly by Ian Desmond.
And while the Nationals had multiple players with offensive success on Friday, Kennedy might have had the biggest impact. He was 4-for-5 with an RBI and two runs.
Kennedy, who was acquired in the offseason to be the everyday second baseman, has played sparingly this season because of quality hitting by Cristian Guzman. But after Guzman was traded to the Rangers on Friday, Kennedy is expected to see much more time in the lineup.
He said he hopes to take advantage of the opportunity and show more of what he can bring as player.
"[Riggleman's] been doing a good job of getting us all in there and trying getting us all at-bats, and hopefully it's all paying off," Kennedy said. "I just got to keep grinding away and keep working."
Similar to the first time the Phillies played in Washington, many of the opposing team's fans packed Nationals Park. Chants and applause for the Philadelphia squad rang through the stadium for most of the game.
But as right-hander Collin Balester took the mound in the ninth inning with a seven-run lead, most of that was quieted. The only real noise came from Nationals fans standing and anticipating another Washington victory.
For Stammen, that's exactly how it should be.
"For the most part, they were a little quieter today, and that's a good thing," Stammen said. "You can just tell around town that there are more Nationals fans than what there was last year. There are more kids wearing Nationals hats.
"As long as we keep winning and putting a good product out there, hopefully those chants won't even be heard when the Phillies come to town."