After considering his options and speaking with friends familiar with the Phillies' situation, including former teammate Brad Lidge, the Houston Astros' staff ace waived his no-trade clause Thursday in order to be dealt for left-handed starter J.A. Happ and two prospects, outfielder Anthony Gose and shortstop Jonathan Villar.
"I'm glad it worked out for both of us. I wanted to go to a contender, and Houston gets some good prospects in return who will hopefully allow them to build a winning team," Oswalt told ESPN.com's Amy K. Nelson.
"The hardest part about it is cleaning out my locker. I've been in the same place for 10 years and having to say goodbye and clean out the locker is the toughest part," he said.
Astros general manager Ed Wade saluted Oswalt for his time in Houston.
"We salute what Roy did for the organization and the contributions that he made here over the course of his career in an Astros uniform and his profile here is significant," Wade said. "At the end of the day, we have to do what's best for the Houston Astros. You can't make a deal like this as a favor to a player. In this case I think we served everybody's purposes with what we did."
Oswalt, who was previously scheduled to start Friday against the Brewers, said he will fly to Washington on Thursday night and start for the Phillies Friday against the Nationals. Coincidentally, Happ was scheduled to start Friday for the Phillies.
The Phillies, who had won seven straight and were 3½ games behind the NL East-leading Atlanta Braves entering Thursday's games, get one of baseball's best right-handers and a pitcher with playoff experience.
"We're trying to do what we can to get back to the World Series and win it," Phillies general manager Ruben Amaro Jr. said. "To have Roy Oswalt, Roy Halladay and additionally Cole Hamels, Joe Blanton and Kyle Kendrick, we stack ourselves up as one of the best rotations in baseball."
Oswalt said his goal was to land with a contender with a chance to play in October.
"I'm at a point in my career where I just want a chance to get back to the World Series like I did in '05 with Houston," he said. "I'm excited to be going to Philly. I love watching Roy Halladay pitch and I think with him and [Cole] Hamels the three of us can feed off each other."
This was the second straight year the Phillies made a major trade for a pitcher in the days leading up to the July 31 deadline. Last season, they got Cliff Lee and he boosted them to another NL pennant -- Lee was then sent to Seattle in a separate deal on the same day the Phillies acquired Halladay.
"I'm excited," Halladay said. "It says a lot that this team is dedicated to winning."
The Astros will pay $11 million of the approximately $23 million that the 32-year-old Oswalt is guaranteed through 2011, sources told ESPN.com's Jayson Stark.
"That was an important part of this," Amaro said. "We wanted to keep some level of flexibility so we can continue to field a championship-caliber team. Yes, the money did make a difference."
Oswalt did not demand that his 2012 mutual option for $16 million be picked up by the Phillies, a baseball source told Nelson. Instead, if either he or the Phillies choose not to pick it up, his $1 million buyout will increase to $2 million.
If Oswalt or the Phillies decline to exercise the option, then the pitcher will make $18 million next season. But both Oswalt and the Phillies must agree on the option for 2012 for him to stay with the team that season, the source told Nelson.
"I didn't want [the option] to be a deal breaker. It wasn't about the money, it was about getting a chance to go somewhere and win," Oswalt said. "I was confident that we could always find a way to overcome that."
He said his conversation with Lidge made him confident that he'd chosen the right destination.
"We played together for seven years and I trust his opinion. He told me that the clubhouse was great and that I was going to love it over there. His opinion means a lot to me," Oswalt said.
Lidge said his conversation with Oswalt took about 20 minutes, but the city sold itself.
"He wanted to know about Philadelphia and the city," Lidge said. "I told him it's been great. Since I've been here, it's been one of the more unbelievable things I've ever seen. I don't think he needed a lot of selling on the idea. He wanted to play for a winner.
"It was probably going to happen anyway. Philly sells itself right now. It doesn't take a genius to figure out what we're doing and what we've done. This is where you want to be if you want to get a ring."
Amaro said Oswalt was "pretty excited" about joining the Phillies.
The 19-year-old Gose is fast, stealing 36 bases and hitting .263 for advanced Class A Clearwater. The Astros then traded Gose to the Toronto Blue Jays for Triple-A first baseman Brett Wallace -- a 23-year-old who hit .301 with 18 home runs and 61 RBIs at Las Vegas.
Wade said Lance Berkman remains Houston's starting first baseman and he's "not anointing" Wallace as the future starter.
"We had a chance to go out and get a guy who's a really solid hitter, who has got a chance to be a really good big league player," Wade said. "But Lance is our first baseman."
Villar, also 19, stole 38 bases and batted .272 for Class A Lakewood.
The Astros had pressed hard for first baseman Jonathan Singleton, considered the team's best minor league hitting prospect, sources told Stark. Singleton is currently at Class A Lakewood in the South Atlantic League, where he was hitting .312 with 18 doubles and 12 home runs as of Thursday.
Happ has made one start after missing three months with an elbow injury. The 27-year-old lefty went 12-4 with a 2.93 ERA and finished runner-up for the NL Rookie of the Year award last season.
"This is all I've ever known," Happ said. "I had an idea driving to the ballpark it might be the last time. It was emotional."
The Astros were off to a National League-worst 14-27 start when Oswalt asked for a trade May 22. As of Thursday, he was 6-12 with a 3.42 ERA.
The Phillies and Astros are not strange trading partners. Wade was Philadelphia's GM from 1998-2005, and Amaro served as an assistant under him. In the 2007 offseason, Wade dealt Lidge to Philadelphia. Lidge was a perfect 48 for 48 in save opportunities and helped the Phillies win the World Series in '08.
"We obviously have a very good friendship and he's obviously been a pretty important person in my life," Amaro said of Wade. "I will tell you this was by no means easy."
For his career, Oswalt is 143-82 with a 3.24 ERA.
ESPN.com senior baseball writer Jayson Stark, ESPN.com baseball writers Amy K. Nelson and Jerry Crasnick and information from The Associated Press contributed to this report.