A few builders say that they prefer OSB over plywood for flooring because the surface is always clean, flat, and knot-free.
Many contractors find OSB to be a more consistent product than plywood or other types of material for floor.
The most important reason for builders to use OSB is money. OSB is cheaper than plywood, so builders can get a bit more profit. When OSB goes beyond flooring use and into wall and roof sheathing, the profit margin increases.