Don't know where the OP is located, and that has bearing on the roof construction.
I recommend removing the entire roof-shingles and sheathing and inspecting the rafters. If the rafters are on proper center, then 1/2 sheathing can be used-5/8 sheathing would only be needed if the rafters are on 24" centers or you have a heavy snow load. I recommend plywood sheathing over OSB. If the pitch of the roof is steep you should consider fir plywood over southern pine as it is less slippery. Proper ventilation under the roof is important.
If you are planning to do this yourself you should thoroughly research the task at your local library.