Re: best material for shed base(plywood or osb)
Plywood, and since you're using it for a base for the shed, pressure treated wouldn't hurt, either that or paint it really good. You don't need T&G, just support the seam with a joist. Also, unless you're going to flash from the shed to the outer edge of the platform, edge flashing isn't going to do anything but hold moisture on those edges.
OSB is garbage, it isn't good for anything, let alone to leave it exposed to the weather.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!