Plywood is made of multiple thin sheets of wood glued together to make a strong board. OSB (oriented strand board) is made of recycled wood chips glued and pressed together. OSB is typically cheaper, more flexible, and more resistant to water. Plywood is generally tougher, more supportive, and more resistant to swelling. Both are comparable materials to use for the subfloor, but plywood is a bit stronger. This makes plywood the better choice for heavier flooring like ceramic or stone tile.