Whilst it's not entirely waterproof, OSB is water-resistant and can be sealed to further enhance its moisture performance. It's as simple as applying a sealant to the surface that requires protection against the elements. A paintbrush should be used to apply the waterproofing solution and it may take 12-14 hours to dry completely. An important thing to remember is that the OSB board should be cut to size before being sealed as any newly cut edge can potentially allow water to come in, if not appropriately sealed. To ensure the waterproofing has worked, it is prudent to pour water on the newly waterproofed surface after it has dried and wait a few hours to determine whether the water has managed to get through the waterproof barrier or not.