I would attach drywall and finish. Since you have no idea what wiring is behind and where, use drywall screws of length to not pass through the other side of the plywood. Or fasten drywall to areas where you know there is a stud underneath.
- Taping & Finishing Drywall Joints
- Inside & Outside Corner Drywall Joints
- How to Repair Small and Big Holes
- How to Build and Finish Drywall Arches
- How to Apply or Repair Texture
Free Tutorials at http://www.drywallinfo.com