OSB has a coating on the outside to make it somewhat water resistant. I read that the makers had to do this because the first batches which didn't have it couldn't survive rain which might occur before roofing went on. It should be fine. I put it on the inside of my shop for the same reasons as you. I didn't want drywall because it's too easy to damage. Once painted white it reflects light fairly well. It's more desert like where I am and I have about R38 in top of it and my shop stays cool during the day and warm in winter.