Assuming 16" OC joists, 5/8" osb or plywood is the minimum that meets current USA codes as I understand it. Now, keep in mind what they call 5/8" is no longer the full 5/8" as it was maybe 20 years ago or so. Personally, I can feel the flex in-between the joists on 5/8" stuff and prefer 3/4". If you have larger spacing on your joists, then you'd probably want at least the 3/4" sheets. Preferably T&G, as the joints along the short edge, especially on the thinner stuff, will rub against each other. I've not looked for 5/8" T&G, and even if it was available, not sure if the tongue would be strong enough to really do anything...choose 3/4" (or thicker) if you can. Ideally, the stuff is installed with construction adhesive on the joists and ring-shank nails or screws.