Looking to protect OSB through the winter
Prime and paint is your best option.  Exterior paint will create a thin latex layer of protection over the surface.  Use primer first to be sure and get a good bond with the somewhat-presealed OSB, then two coats of a quality exterior (porch paint should be great) paint.  Two gallons of paint and two gallons of primer should be plenty for two coats.  And remember that a coat of primer and a coat of paint is better than two coats of paint.

Be sure and and get very good coverage on the edges.  I might go so far as to caulk the seams with some cheap caulking to prevent water from soaking to the cracks.