The nightcap was lost when O'Sullivan allowed three two-out singles in the sixth. Mario Hollands inherited two runners, both of whom scored on rookie Tommy La Stella's double.
Two Ryan Howard errors ignited the first defeat, a 10-3 Braves thumper.
The Phillies reached the halfway point overcome with humiliation. They are 36-45, three games worse than last season, and on pace for 72 wins. They dropped to nine games under .500 for the first time since June 15. The good will from a five-game winning streak has long evaporated.
"I still have faith in this squad," Ben Revere said. "A lot of things can happen in this game of baseball. We have the guys to do it. We're not that far behind. We can go on a roll at any time."
In the first game, fans booed Howard when he failed to glove two routine balls in the first. They screamed at Antonio Bastardo after he allowed five runs in the span of six batters. In Game 2, they heckled Domonic Brown when he pinch-hit and could not advance a runner. And, after all the atrocities against baseball, they directed venom toward mop-up man B.J. Rosenberg.
The Phillies have used more players than any other team in baseball. They have not yet fielded a consistent, winning combination. More changes are soon to come. Their 310 runs scored through 81 games represent the franchise's lowest output since 1997. That team, which finished 68-94, scored 286 runs in the first half.
"We have to do things differently; we definitely have to have more opportunities to score runs," Sandberg said. "We have to be more consistent putting the pitching and defense together and play more solidly, more consistently."
The lineup's failures were prominent in the second game. The Phillies placed a runner in scoring position with less than two outs in three innings - the fourth, sixth, and seventh. They did not score.
In the afternoon game, even when they mustered a quick lead, stupid mistakes conspired against them. Atlanta slapped three straight routine grounders at Howard in the fourth inning. One hopped between his legs. He charged another dribbler, could not glove the ball, and then fumbled it toward the mound. He scooped the third one, stepped on first base, and mock cheers greeted him.
"That changes the whole game," Sandberg said.
The Braves tied it. They did not trail again all day.
Both Phillies starters stumbled in the sixth inning. O'Sullivan allowed one run in the first five innings of the second game. The 26-year-old righthander wavered in the sixth when Justin Upton and Ryan Doumit laced back-to-back singles. Sandberg stuck with O'Sullivan. Chris Johnson smashed a first-pitch fastball to right for a run-scoring single.
Upton, in the first game, drilled a Roberto Hernandez elevated sinker for a two-run double in the sixth to push the Braves ahead. Those were Hernandez's lone earned runs in seven innings.
Two reserves, Tony Gwynn Jr. and Koyie Hill, injected more life into the offense than anyone else. Gwynn's first-inning triple in the afternoon game was his first extra-base hit since April 26, a span of 66 plate appearances. It snapped a 0-for-23 skid. Hill, the team's fourth-string catcher, doubled in the second. Hernandez notched his first career RBI with a single to right.
"I'm doing better," said Hernandez, who was hitless in his first 40 career at-bats but is 2 for his last 4.
Bastardo laid waste to the first game in the eighth inning. Luis Garcia, summoned as the 26th man for this doubleheader, replaced Bastardo. He fired his first pitch to the backstop for another run. How fitting.