How to get oil stain off plaster wall?

The only thing that will remove oil is a solvent or a detergent. Bleach and water will not help at all and ammonia will be only slightly effective. It is unlikely that you will remove all the oil, but you can improve the situation. You could use a good detergent like Dawn dish liquid, or an even better product like Dawn Power Dissolver (found in the dish detergent isle of the grocery store. Since it is on a vertical service, it would be hard to apply a product to draw out the oil, like kitty litter. After you have removed all the oil you can, then I would go to the paint store and get a good oil base, stain blocking primer and prime the spot. You can follow that with either latex or oil base paint. Be sure to read and follow the directions on the primer and paint. If you are in doubt, go to a dedicated paint store (not HD or Wal-Mart) and ask for product advice.