Drywall is supposed to be installed across the studs anyway (the grain of the paper is stronger that way), and already 4' wide. I think it may be easier to just cut out the bottom 4' and replace it. Painted OSB will likely always look like painted OSB, and then you'd have the edge to deal with. It would be a shame to paint it all and then find out that the stains bleed through, so I'd opt for a stainblocker. You might get lucky without it.