O'Sullivan (knee) was activated off the disabled list Thursday, and outrighted to Triple-A Pawtucket, Evan Drellich of the Boston Herald reports.
O'Sullivan had been on the DL for just over a month after dealing with left knee tendinitis. He recently completed a rehab assignment with Pawtucket, but was roughed up pretty good, allowing six runs on six hits in three innings. The team decided to keep him with the Triple-A team, but O'Sullivan will likely come back to the majors before the end of the month.