What’s the best substrate for a painted wood floor?

You’re right to be hesitant about painting chipboard (officially called oriented strand board – OSB – in the trade), since it’s too rough for good results. But smooth plywood installed over top makes a great surface for paint. I’ve seen and walked on painted floors several times and they look terrific. The least expensive option involves fastening down some 1/4-inch thick underlay plywood (usually made from a tropical hardwood called luaun) using deck screws driven into the floor joists. Just follow the line of existing nails in the OSB, and you’ll hit the joists. You could drive the screw heads flush with the floor surface and leave it at that, or fill them with auto body compound and sand the surface flat. The most successful painted floors I’ve seen have all been coated with exterior-grade latex paint. The best one has a textured, mottled appearance created by sponging several colours onto a white base coat.