What predators are you proofing against? Curious, actually. Knock on wood (not the roof) I don't think we've ever had a predator in one of our coops. We've always had at least one dog and a doggy door that never closes! (flaps, of course. - home made with shower pan liner flaps - cheap...
Which is another good reason to pitch your metal so it runs down hill and out.
It is a Chicken Coop - birds roost in trees in the wind and the rain and even in the snow a little condensate's not going to bother them at all. We have bird the choose to roost in the peach tree even tough the coop...
If you're using 'new metal' the OSB would be wasteful IMHO. I used 1x4 purlins 24" OC in both my metal roofing projects and OSB or Plywood Sheathing for my shingle roofed projects.
Years gone by - no complaints or issues with the metal. Chickens don't need much in the way of shelter.