The Nationals have only gone from 27th in runs (April) to 16th (May) despite their improvement in OPS. They've needed those extra runs to keep up (9-6 in May) because their pitching has done downhill this month, sliding from #1 in team ERA in April to 13th in May.
One of the main reasons for the team OPS improving so much is that Roger Bernadina has an OPS of .995 in May (31 AB), up from .509 in April (42 AB). Rookie phenom Bryce Harper was called up on April 28th, and has struggled a bit (.222 BA, .726 OPS), although he has 2 HR in the last 3 games. It's possible his presence has provided a jolt to the rest of the lineup, even if he hasn't taken off himself yet.