Ohlendorf (3-0) pitched five innings of two-hit ball before Christian Yelich led off the sixth with his second homer, ending the starter's outing. It was Ohlendorf's second start since he was sidelined by right shoulder inflammation.
Tanner Roark, Drew Storen and Tyler Clippard each pitched a hitless inning before Rafael Soriano finished the four-hitter for his 34th save.
Washington kicked off a six-game homestand with its sixth victory in the last seven games. The Nationals began the day eight games behind Cincinnati for the second NL wild-card spot.
Yelich finished with two hits for Miami, which has lost six of seven. Nathan Eovaldi (2-5) allowed two runs -- both in the first -- and eight hits in six innings.
The victory came in the opener of a stretch of 19 consecutive games for Washington against the Marlins, Mets and Phillies, all NL East teams below .500.
"Now's the time," manager Davey Johnson said before the game. "I still think we need to win close to 90 games (to make the playoffs). So every game we need to play well, we need to pitch well."
Ohlendorf and the bullpen took care of the pitching, and the lineup did just enough damage in the first.
Ryan Zimmerman and Bryce Harper had consecutive one-out singles, putting runners on the corners. Zimmerman scored on Jayson Werth's groundout and Desmond singled in Harper to make it 2-0.
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Washington totaled seven baserunners after the first, but couldn't add to its run total. All nine Nationals hits were singles.
Ohlendorf retired his first nine batters before Yelich singled to start the fourth.
Giancarlo Stanton followed with a double and Logan Morrison was walked to load the bases, but Ohlendorf fanned Ed Lucas and got Justin Ruggiano to ground into a fielder's choice.
NOTES: Denard Span and Harper extended their hitting streaks to 10 and nine games, respectively. ... The Nationals recalled RHP Ryan Mattheus from Triple-A Syracuse and optioned LHP Xavier Cedeno to their top farm club. ... Marlins manager Mike Redmond said RHP Jose Fernandez has two or three starts remaining before the club shuts the 21-year-old down. Fernandez has thrown 152 2-3 innings and could approach his team-imposed 170-inning limit during his next two starts. ... Johnson said an X-ray of LF Harper's left foot was negative after he fouled a ball off his leg on Sunday. ... Johnson said "probably not" when asked if LHP Ross Detwiler, on the DL since early July with a back strain, will return before the end of the season. Detwiler has begun playing catch, but hasn't thrown off the mound. ... The Marlins reinstated INF Placido Polanco from the 7-day concussion DL. ... Miami RHP Henderson Alvarez (2-3, 3.86 ERA) opposes Stephen Strasburg (6-9, 3.00 ERA) in Wednesday's game.
Press; By HARVEY VALENTINE]
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