Vegetable oil does not help rabbits like it does for dogs and cats. It should be avoided if possible as an ingredient in the food. The best way to keep things moving through a bunny's digestive tract is to keep them hydrated and with plenty of fiber. Oil will not help.
"E. Petroleum-based laxatives: use with caution.
Laxative-grade mineral oil or commercial products such as Laxatone or Petromalt do not affect intestinal motility. Some veterinarians prescribe them in the hope that they might help to slide dry, impacted matter through the intestine more easily.
Note, however, that if the intestinal contents are severely dehydrated and brick-hard (yes, we have seen this!), a coating of vaseline-like substance over them will merely impede their re-hydration and make it more difficult for the mass to break up and begin passing normally. For this reason, it is probably wise to concentrate on re-hydrating the intestinal contents before using petroleum-based laxatives, if they are to be used at all.
Note also that whereas malt-flavored remedies in a tube are often preferred by the bunny, some vets believe that their higher viscosity may actually contribute to holding a mass of impacted food together, especially if the intestinal contents are dehydrated. Unscented, laxative grade mineral oil is less viscous, and may be more effective. Always administer such substances with care so that the bunny does not aspirate (inhale) any. Petroleum-based laxatives should not be given daily or long term, as they can impede the absorption of important, fat-soluble vitamins."
http://www.bio.miami.edu/hare/ileus.html