Left to a battle of the bullpens, Olsen's outing was the one that went for naught.
With Olsen out of the game, the Florida Marlins took advantage of Washington relievers Ron Villone and Julian Tavarez for two eighth-inning runs that produced a 4-2 victory over the Nationals.
"It was encouraging to see Olsen pitch that well," manager Manny Acta said after the Nationals lost to the Marlins (39-39) for the eighth consecutive meeting, dating back to last season.
"Our bullpen -- it is what it is," Acta added. "We've had trouble there for the most part this year. But we've got to take the positive out of it."
Olsen, on the disabled list since May 17 with shoulder tendinitis, scattered six hits over seven innings -- matching his longest outing since April 18.
Though he struggled a bit early on with getting ahead in the count, the onetime Marlins pitcher ran into serious trouble only in the third inning, when his former team touched him for four hits and two runs.
"I made some really, really bad pitches in the third inning," Olsen said. "But then I kind of got on a roll after that in the fourth, fifth [and] sixth. I was able to place my pitches where I wanted them. But I still made some awful pitches there in the third. So there's always room for improvement."
Olsen rolled through 1-2-3 innings from the fourth through the sixth, where he notched five of his seven strikeouts. He also retired the first two batters of the seventh before Ronny Paulino ended the streak with a single back through the box.
The lefty threw first-pitch strikes to 16 of the 26 batters he faced, including 15 of his last 17.
"He started out a little bit off with his command," Acta said, "but then as the game went on, he got better and better. It was a tremendous outing. You couldn't ask for anything better."
Acta, though, couldn't say the same about his relief corps. Villone (3-5) opened with three straight balls to pinch-hitter Wes Helms, battled back to a full count, then watched as Helms laced a sinker just inside the right-field line for a double.
Pinch-runner Alejandro De Aza moved to third on Chris Coghlan's sacrifice, and Emilio Bonafacio got just enough of another Villone sinker to lift a fly to center field that brought home De Aza without a throw.
"It's a game of inches, and you saw that again tonight," said Villone, in part blaming his struggles on a divot in his usual landing spot on the mound. "I just need to adapt, and tonight I didn't do that at all."
Acta said: "His command hasn't been there. He just needs to go back to making pitches instead of throwing hard, hard, hard."
Tavarez looked worse, though the damage came out equal. After he was greeted by a Hanley Ramirez double, the right-hander intentionally walked Jorge Cantu before conventional walks to Dan Uggla and Cody Ross brought Ramirez home.
Nolasco (5-6) picked up the win by allowing just four hits over eight innings. Three of the hits came in Washington's two-run third, when Ryan Zimmerman led off with a homer and Wil Nieves singled home Josh Willingham.
After the Nationals (22-52) touched Dan Meyer for two hits in the ninth, Leo Nuñez notched his third save by getting Willingham on a fly to center.
"Nolasco was outstanding," Marlins manager Fredi Gonzalez said. "He gave us eight innings, and we were able to get to those guys in the bullpen right where we wanted them. Scotty Olsen was outstanding, too."
Olsen's velocity was a concern when he went on the disabled list, but the lefty continued to hit the low 90s throughout Monday's outing.
"I'm not Jamie Moyer -- I can't really be hugely successful throwing 82 or 83 [mph]," Olsen said. "The exercises and new routines that the training staff and physical therapists got me on here -- it's a real credit to them. Every start I've made to this point has put me at 88 to 92 [mph]."
Acta said: "I don't think anybody's doubting his ability. We've seen him in this division enough. He went out there today and gave us a good outing."
The start of the game was delayed 37 minutes by thunderstorms, which reached Land Shark Stadium just as the Nationals were set to take batting practice.
Jeff Shain is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.