O'Sullivan is scheduled to start Sunday's game against the Angels, Evan Drellich of the Boston Herald reports.
O'Sullivan, whose Triple-A start on Thursday was skipped in preparation for this move, made 11 starts for Triple-A Pawtucket this season and sparkled, posting a 6-2 record with a 2.79 ERA over 71 innings. He'll now have a chance to carry over his minor league production Sunday as he faces off against Matt Shoemaker and the Angels.