In new home construction, the roof sheeting and side sheeting are grade specific OSB. But the floors are all plywood. Plywood has greater liner strength as the grain is crossed over from different directions with the different plys which makes it better to walk on and gives better support across the floor joists.

http://www.woodbin.com/ref/wood/strength_table.htm

Elasticity is a property of wood in which strains or deformations are recoverable after an applied stress is removed, up to a certain level of stress known as the proportional limit. Below this point, each increment of stress will produce a proportional increment of strain (the stress/strain ratio is constant) and the wood will return to its original position once the stress is removed. Beyond the proportional limit, each increment of stress will cause increasingly larger increments of strain (as failure is approached) and removal of the stress will only result in a partial recovery of the strain.