10 most extreme places on Earth. Used for sub floors, exterior walls, and roof decking, OSB sheathing is a structurally-engineered board that is made up of compressed wood strands. These strands of wood are arranged in layers that are bonded together with resin, which helps produce a strong product.SB sheathing thickness can range from 3/8 inches to 1 and 1/8 inches. 9 feet and 10 feet long osb sheathing is made but you might have to request it from the manufacturer beforehand as it might not be readily available on hand.