OSB is a fairly stable material for nailing and gluing. Whether it is an accceptable substrate without additional underlayment really depends on the method you use to fasten the floor and recommendations of the flooring manufacturer. I don't think you ever read that a subfloor should be removed here. The issue is really wheter you need to add an underlayment. It may be acceptable to simply install the floor on your subfloor. Adding an underlayment can provide an improved nailing substrate and surface, and works to protect the subfloor for future changes. As far as I know you don't need to add a great deal of thickness, but 1/4" plywood would be the minimum. I have usually used a 1/2" A-C exterior plywood with 15#felt barrier.