what material do you use to paint with acrylic over oil
It depends on the type of paint used in the existing painting. If it is an oil painting, you will need to use oil based paint. If it is a water based medium, like acrylic, use a water based paint. Gesso will work well for this. It is inexpensive, opaque and will help level the texture of the existing painting. You may want to lightly sand the painting before painting over.