Houston's ace, who was placed on the DL on July 12 for a strained left hip abductor he suffered on June 30, said he threw fastballs and changeups during the 10-minute session.
He said he felt "good" and had no discomfort.
"I didn't throw 100 percent," Oswalt said. "I threw about 75 to 80 percent, and it feels pretty good. I'll know a lot more Friday, when I throw 100 percent and see how I feel."
Oswalt, who threw on flat ground the day before, is expected to pitch on July 28 against the Reds in Houston, his first start since a one-inning, 17-pitch outing on July 11.
Astros manager Cecil Cooper said Friday's bullpen session will be the determining factor in deciding if Oswalt is ready, and Oswalt said there is no reason to rush his return.
"The biggest thing is no matter how hard you push yourself in the bullpen, once you get a hitter in front of you, you always crank it up a little bit more," he said. "The best thing about it is my mechanics are good enough to make my body take all the load instead of my arm. I'd rather have body parts instead of arms be sore. Once your arm starts hurting, it's hard to change that."
Krysten Oliphant is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.