Wear safety gear. Whenever you work on a ceiling, safety glasses are an essential to protect your eyes from drips and dust. Put on gloves as well to protect your hands.
Apply a paint remover to a rag and gently rub the oversprayed area. A commercial paint remover may work if the overspray is light and relatively fresh. Wipe the area down and scrub stubborn spots if necessary.
Sand away the paint. If the paint remover did not work, use a fine grade of sandpaper and go over the oversprayed area. Work as lightly as possible so that you can avoid ruining the finish on the wood ceiling if you can.
Remove stubborn spots. If the ceiling wasn't finished or if the paint overspray was particularly heavy, you will need to refinish this area. Use a medium grade of sandpaper to get down past the finish and the paint. Go over this area with a fine grade of sandpaper to smooth it out. Wipe away any dust.
Refinish the spot. If you didn't have to go below the protective coat, a light coating of clear polyurethane will seal up the spot. If you did go into the wood, stain it with a matching wood stain. Let this dry, sand over it lightly and coat it with polyurethane.