I take it you are using OSB and not obs as you stated. OSB is Oriented Strand Board, meaning the chips which make up the composite are oriented so that their grain runs mostly in one direction, giving strength in that particular direction.
I've seen people using Birch furniture grade Plywood for bench work: Menards price for 1/2 inch thick 27.02 for a 4X8 sheet. I used 1/2 CDX Sheathing plywood on my layout: with a Menards price of $14.91 per 4X8 sheet. You are using OSB: if 1/2 inch, Menards has a price of $9.35 per 4X8 sheet. That is a significant savings in material for you. I can't see a reason to use Furniture Grade Plywood. Maybe someone can give a sound reason. At the time I started building my layout, much of the "Building a Layout" articles in the Model railroading press talked about using regular plywood, so that is what I used. When I asked about stability, I was asking about shrinkage and expansion, not saging or warping. I would probably go with OSB, now and enjoy the cost savings!