Oswalt (9-13) won his third consecutive outing, scattering five hits and a walk while fanning eight for the Phillies, who recovered from an 8-1 loss on Saturday to stay in front in the National League Wild Card race while remaining 2 1/2 games back of the East Division-leading Braves.
Scott Olsen (3-6) was touched for four runs on nine hits in a six-inning start for Washington, which went 0-for-9 with runners in scoring position to lose for the fourth time in the last six games.
After Oswalt struck out Adam Dunn and Ryan Zimmerman to leave Nyjer Morgan on third in the top of the first inning, the Phillies manufactured a run in the bottom half when Jimmy Rollins led off with a double, moved to third on a groundout and raced home on Polanco’s sacrifice fly.
Oswalt stranded a runner on second in each of the next two innings, then singled to begin the home third, advanced on a groundout and scored on Polanco’s base hit up the middle.
Washington again squandered a scoring chance in the fourth as Mike Morse and Adam Kennedy both whiffed to leave Roger Bernadina 90 feet away.
Wilson Valdez’s bases-loaded single in the sixth scored Jayson Werth and Shane Victorino to double the Phillies’ lead.
The sky opened after Oswalt retired his seventh consecutive hitter to end the seventh. When play resumed following a one-hour, 44-minute rain delay, Ibanez took Craig Stammen deep over the right-field wall. Rollins singled ahead of the blast.
Ryan Madson pitched a 1-2-3 eighth, and J.C. Romero did the same in the ninth to finish off Philadelphia 15th shutout of the season, the most for the club since 1967 (17).
The Phillies, winners in eight straight day games, have won eight of their last nine series…Washington is 0-14-2 in its last 16 road series…Olsen has lost his last four starts…Victorino had two doubles in the win…Morgan had two of Washington’s five hits…Nationals rookie hurler Stephen Strasburg, who exited Saturday’s game with an apparent arm injury, felt no discomfort in going through a normal workout routine ahead of his scheduled MRI exam back in Washington, D.C. on Sunday.