Oswalt lasted 3 1/3 innings, giving up three earned runs and five hits with one walk. He threw 85 pitches, 55 for strikes.
"Breaking pitches weren't there tonight," Oswalt said. "Couldn't get my curveball over, and my slider was kind of flat. The biggest thing is getting the pitch count up. We got up to around 90 pitches, but it was a little quicker than I wanted it to be."
Oswalt got through the first two innings with no damage but ran into trouble in the third. The leadoff man reached after striking out on a wild pitch. A one-out double and back-to-back two-out hits led to a three-run inning.
Oswalt is scheduled to make his next start Sunday with Triple-A Round Rock, where he made his first two starts.
In his first start, Oswalt threw 34 pitches over two scoreless innings. His second start was not as smooth. In four innings, Oswalt threw 56 pitches and gave up five runs on six hits and a walk to go along with four strikeouts.
Oswalt said his arm feels good and he feels strong, but he wants to be more consistent with his breaking balls.
"Out of the last three, this was probably the worst one stuff-wise. I really didn't have good stuff," Oswalt said of Tuesday's start. "I felt OK. I felt pretty strong. I just couldn't get my breaking pitches over."
The three-time All-Star knows he has work to do before he's ready to step into the Rangers' rotation.
"The biggest thing is I just need to get deeper in the game," Oswalt said. "Four innings isn't going to help a big league club out distance-wise. Pitch count is going up. Next start, I think we're going to go around 100-105 and see where we're at then."
"I'm not going to rush there and not be ready when I get there," Oswalt said. "I'm going to go as fast as I can to get there. I think they wanted me to go around six or seven (minor league) starts, and I'm trying to do it in four. We'll see what the next start looks like and base it off that."
The Rangers are Oswalt's third organization. The Houston Astros drafted the right-hander in the 23rd round of the MLB Draft in 1996. He made his big league debut in 2001. Oswalt's 10-year career with the Astros came to an end in 2010 when he was traded to the Philadelphia Phillies, where he spent the previous two seasons.
David Collier is a contributing writer for ESPNDallas.com