It depends on the rest of your team.
Are you starting Lynch every week? If you are, giveing up Lynch and plugging Grant in there is a definate downgrade in my opinion. Without Favre GBs running game doesn't get the open lanes it used to.
If you have RB depth, and are weak at QBs, it may be worth it, but I don't think O'Sulivan is a legit every week starter anyway. Depending on the size of your league rosters, you may be able to plug in QBs off the waiver wire (Orton, Russell, Frerotte this week, etc) with good matchups and get about what you'd get out of O'Sulivan.
Personaly I'd keep Lynch, dump Bulger outright, and grab either Frerotte or even Charlie Frye to play this week if you need a QB. They may do better this week than O'Sulivan will against Philly anyway.