O'Sullivan was called up by the Red Sox on Friday, Michael Silverman of the Boston Herald reports.
He will provide some depth to the Boston bullpen, but O'Sullivan figures to head down next week to make room for Eduardo Rodriguez (knee), if he makes it that long. The 28-year-old O'Sullivan posted an ERA north of 6.00 with the Phillies the last two seasons, and he owns a 4.23 ERA and 1.36 WHIP in parts of eight seasons at the Triple-A level.