If roofing membrane+shingles will be added, do they need to have the OSB underneath? or can it be something thin and light like MDF? or HW cloth? sorry if this is a stupid question.
There has to be wood underneath to have something to nail to.
OSB is bare minimum. Plywood would be preferred. Roofing nails are a ring shank nail which has ridges on the sides of it to help prevent pull out as opposed to a smooth shank nail.