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PHILADELPHIA (Reuters) - The Philadelphia Phillies acquired ace Roy Oswalt from the Houston Astros on Thursday, adding another All-Star arm to their pitching staff as they prepare for another World Series run.

Oswalt, a two-time 20-game winner who has spent his entire 10-year major league career with Houston, joins a stellar starting rotation that already includes former Cy Young winner Roy Halladay and Cole Hamels.

Second on the Astros all-time win list with 143, Oswalt had asked to be traded to a contender after struggling this season with a 6-12 record.

The two-time National League champion Phillies sent pitcher J.A. Happ and minor leaguers outfielder Anthony Gose and shortstop Jonathan Villar to Houston in exchange for the 32-year-old right-hander.

Houston then shipped Gose to Toronto for minor league first baseman Brett Wallace.

"It's exciting for sure," Oswalt told baseball's official website (www.mlb.com). "I think it works out for both of us. Houston's getting good prospects and another pitcher, and I'm getting to go to a great team.

"From the very beginning, I said I wouldn't accept it unless it worked out for both of us, and I think it worked out."

The Phillies enter Thursday night's game against Arizona riding a seven-game win streak but trail Atlanta by three games in the N.L. East.

After cleaning out his locker, Oswalt was expected to fly to Philadelphia later Thursday and was scheduled to start for the Phillies on Friday in Washington against the Nationals.

"I think probably the toughest part was packing up my locker knowing I started here and have to leave now," Oswalt said. "It's going to be good for the organization, and I think it will be good for me, too.

"You've pretty much got an All-Star at every position.

"I talked to (Phillies pitcher) Brad Lidge last night about the clubhouse, and he likes it a lot up there with guys like Jimmy Rollins and Shane Victorino and some other great guys.

"Overall, it came down to Houston getting something for me and I was getting to go to a contender."

Oswalt is a three-time All-Star and was the MVP of the 2005 N.L. Championship Series when the Astros beat the St. Louis Cardinals in six games to reach the World Series.

(Writing by Steve Keating in Toronto, Editing by Steve Ginsburg)