I'm with InspectorD, Be careful about painting OSB. One side maybe both sides have a coating intended to protect the material from moisture. Paint can blister off of the protected side and on unprotected OSB, the material can blister and flake off, the edges can turn and raise off of the surface.
OSB can be painted, but I use a primer first, then a scraper to remove the loose pieces, sand it then prime it again. Then you can use block filler or whatever you want without worrying about flaking and blistering.
Scrape and sand the loose first and use an oil base primer and let it dry for a day or 2. Then paint.
I have a wide exterior door made of OSB on my shop that I primed with an oil base primer 4 years ago and haven't had any flaking yet.
I really need to paint it with a finish coat