I never liked OSB for walls and ceilings inside a building. It looks bad to me. But when I painted OSB for projects, I'd lay it down on saw horses. Pour KILZ out of the bucket onto it and spread it out with a roller. Then I'd pour paint out of the bucket and spread it out with a roller too, for the final coat. It dries pretty quickly if you do it out in the sun in the summer. Just dump it out and spread it with a roller.
