Absolutely NO on "ospho" .....
Dry them, a wee bit of oil... but NO ospho. If intergranular corrosion has begun (in the at the roots of the threads, etc.) Ospho or any other 'acid' will make it WORSE. A thin coat of oil or a thing coat of wax is air-permeable; the 'best' is 'dry'. If you see a lot of 'rust bloom' at the base of the nuts/washers, then consider in the near-term future to drop the keel and 'inspect the bolts'.