Oswalt, 35, who signed with the Rockies on May 3 and spent the time since then building for Thursday, saw that his strikeout total was a double-edged stat. He reached 101 pitches in five innings and left the game.
"It's hard to say this, but really too many strikeouts kind of got my pitch count up, and I was trying to get deeper into the game," said Oswalt, who fanned 12 on Sept. 9, 2001 -- his rookie year -- against the Brewers.
Thursday was Oswalt's first Major League appearance since last Oct. 2 and first start since last Sept. 9 and will go down as a nice statement -- there are no health questions, and the power is there. But the reeling Rockies, who have matched their longest losing streak of the season by going 0-4 to start a nine-game road trip, need more than strikeouts from a new starter.
The Rockies dropped to .500 for the first time since April 2. They are 2-5 since star shortstop Troy Tulowitzki sustained a rib injury that is expected to keep him out four to six weeks.
Oswalt's debut was spoiled in part by Nationals starter Jordan Zimmermann (10-3), who held the Rockies to six hits and no earned runs with nine strikeouts of his own in eight innings.
Michael Cuddyer extended his career-best hit streak to 18 games with a first-inning single, but Zimmermann did not allow the Rockies much offensive success beyond that. DJ LeMahieu's RBI single off Zimmermann in the eighth plated an unearned run that prevented the Rockies from sustaining their second shutout in four games.
"Anybody that's throwing 98 [mph], it doesn't matter what the breaking stuff is; it's going to be tough to hit," said Rockies first baseman Todd Helton, who managed two hits.
One of the Nationals' hits off Oswalt was Ian Desmond's 11th homer of the season, with one out in the second, and another was Adam LaRoche's fourth-inning, two-run triple. But it looks as if the Rockies will get plenty of mileage out of Oswalt's fastball, which topped out at 94 mph and was the put-away pitch on all but one of the strikeouts.
"There was plenty of stuff there, plenty of arm," Rockies manager Walt Weiss said. "He had a really good fastball. He got a few tough breaks. His line could've been better than what it showed."
Oswalt, however, said his changeup was not sharp. One resulted in his first strikeout, when he froze Denard Span. But another resulted in Desmond's homer.
"I felt pretty well, but I've got to work on command with my offspeed pitches a little more," Oswalt said. "Being a little excited probably made the changeup a little worse than it should be."
"The first inning couldn't start fast enough today."
Still, Oswalt, 35, signaled that he might be ready to anchor the staff the way Jon Garland, since released, and Jeff Francis, who accepted an option to Triple-A Colorado Springs on Thursday, could not.
The Rangers brought Oswalt in under similar circumstances last year, but he was in and out of the rotation and ultimately finished with a 5.80 ERA. He sat out Spring Training and the first month of the season waiting for offers. At least one team offered him a chance to close, but the Rockies offered him a chance to do what he had done his whole career, and in the National League.
The Rockies believe things will be different from last year because Oswalt is more ready. He stayed at Double-A Tulsa for five starts (2.16 ERA), after three extended spring training outings. Oswalt had back issues with the Phillies in 2011 but insists he is healthy, and he looked it Thursday.
"I honestly wasn't expecting him to be in the same form he was before he left," the Nationals' Desmond said. "I'm sure he'll say he made a couple mistakes, and every pitcher does, but he did an unbelievable job."
Oswalt topped out at 102 pitches in his last start in Tulsa and was not in position to go much longer. But the Rockies have not let any starter throw many more than 100 this season. Oswalt said the Rockies told him he was under the same system, and he expects to thrive under it.
"I feel like I've got the same stuff I had in 2010," said Oswalt, who posted a combined 2.76 ERA with the Astros and Phillies, including a 1.74 while going 7-1 in 12 starts with the Phillies after being traded for the playoff run.
Oswalt trailed, 1-0, in the fourth Thursday and had two out and two on when Kurt Suzuki dribbled an infield single that shortstop Jonathan Herrera partially blocked but could not keep in the infield. LaRoche scored from second, although catcher Yorvit Torrealba thought he made the tag in time.
In the fifth, LaRoche's one-out triple went to right-center and bounced just in front of and then eluded center fielder Tyler Colvin, playing the position and leading off as Dexter Fowler missed his third straight start after being hit by a pitch on his right ring finger a week ago.
The Nats' Steve Lombardozzi doubled in the sixth off Adam Ottavino and scored on Zimmermann's sacrifice fly.