24" OC is what I have in my pole barn, NOT my house. I would worry about that more than the sheathing.

In terms of OSB vs. Ply, I would only use OSB if it is part of the ZIP (http://www.zipsystem.com/)system which has a plastic coating on it as part of the weatherproofing.

Even with the ZIP system, we used felt paper on the roof and housewrap on the walls.