Is it OK to reheat hash browns the next morning?
Frozen hash browns like that are already pretty much cooked. When you bake them, you're really just heating them up and crisping the outside.

If you cook some then save them, they're going to lose a lot of that crispiness no matter what you do. If you eat them cold they'll be, well, cold. If you heat them in the microwave, they'll soften even more. If you still like them, go for it.

Safety-wise, it's fine. You do need to keep them in the fridge, as with pretty much any other leftover food.