Wow, I just realized that the first part of this blog is entirely missing, I apologize. I wrote it, but it somehow disappeared. Anyway, OSB (stands for "oriented strand board") is a builiding sheet material that is commonly used in building new homes. It is used on roofs, exterior sheeting, and ... for sub-flooring. Builders like it because it is cheaper than plywood, therefore it has pretty much taken the place of plywood in the building industry. There is however, one precaution when using this material; when it is used for sub-flooring in new construction applications. It should be kept relatively dry during the construction process, and before the roof is put on to help protect it. Why should it be kept dry? Because of this materials tendency to SWELL up when it gotten overly wet, and not allowed to dry properly. Because of it being used for flooring, which is supposed to installed basically flat and level, if it rains the material can become saturated, swell up to 1 1/2 times its original thickness, and also, lose overall strength and durability.
Therefore, precautions should be taken to ensure that it stays dry. If you build, and it rains before the roof has been applied, then the sub-floor should be broomed dry afterwards, and if there is any remaining puddles, a small (3/8 inch) hole should be drilled in the floor to help it drain out. This will help prevent swelling and loss of strength in the material.
OSB that has swellled considerably should be replaced. If it is not, then strength of the floor will be lost, the floor will have high spots, and probably be noisy to walk on. I have seen several builders just ignore this, and do nothing about it.