Depending on how many rooms need to be covered with Gypsum board, it may be easier to have a local Drywall guy do the hanging & finishing, and you paint. Figure around $9 a sheet for 1/2". Keeping the OSB and covering with 1/4 is not something I would do. Get three bids for the house and go with the best one, but not the lowest, unless you know the guy/gal & can really trust their work.
Figure since the walls are open, you can update the plumbing, Electric, pull some Coax, Telephone, Network wiring, and put in better wall insulation