Can oil base primer be used as a finish coat over wallpaper?

Not a good idea. The adhesive is usually water based and will liquefy when the primer is applied, resulting in less adhesion of the primer and a streaky, uneven or textured finish.

It is best to take the time, use wallpaper stripper in warm water and remove the adhesive. Then rinse down the walls with clear water and allow it to dry before applying the primer.