Scott Atchison replaced Niese with two outs in the sixth and stranded two inherited runners.
Ramos' RBI single against Atchison in the seventh made it 5-3.
"We finally got some hits with runners out there," Johnson said. "It took a while."
Murphy followed Eric Young's leadoff double with a two-base hit for New York's first run and scored one out later on Duda's single.
Another RBI single from Duda, who started for Davis, made it 5-2 in the seventh.
New York went ahead 3-0 as de Dekker's first major league home run landed in the second deck of the right-center field stands.
Ohlendorf pitched five innings, allowing four runs and nine hits. He had a 2.82 ERA in four previous starts this season.
Notes: Zimmerman had three hits. ...The Mets, with their rotation depleted by injuries, have signed RHP Aaron Harang to a minor league contract and assigned him to Triple-A Las Vegas of the Pacific Coast League. Harang was 5-11 with a 5.76 ERA in 22 starts this year for the Seattle Mariners. Overall, he has pitched 12 years in the majors, going 110-115 with a 4.29 ERA. ... The Mets also called up three players, including RHP Vic Black, who was acquired on Aug. 27 from the Pirates in a deal that sent OF Marlon Byrd and C John Buck to Pittsburgh. ... Washington also recalled RHP Ian Krol from Triple-A Syracuse and catcher Sandy Leon from Double-A Harrisburg. ... RHP Daisuke Matsuzaka (0-2, 8.68 ERA) opposes Paul Maholm (9-10, 4.37) as New York opens a three-game series at Atlanta on Monday. ... Washington plays at Philadelphia on Monday as RHP Stephen Strasburg (6-9, 2.96 ERA) faces LHP Cole Hamels (6-13, 3.58 ERA).