Personally, I would not use an oil on ivory. It tends to darken the ivory. I would apply several coats of refined beeswax or carnuba wax. I have heard of spraying it with clear laquer too.
From the Smithsonian Museum Conservation Institute:
THE CARE AND HANDLING OF IVORY OBJECTS
Important things to know about ivory
Ivory - as distinguished from bone, antler or horn - includes the tusks and the unusually large or projecting teeth of animals such as elephant, walrus and several species of whale. It consists of dentine, made up of components which are both organic (to provide the capacity for growth and repair) and inorganic (to provide rigidity and strength). Identification of the type of ivory is based on the composition of the dentine and its specific growth structure.
Ivory is very reactive to its environment. It bleaches when exposed to light but the most severe changes are linked to changes in relative humidity and temperature. Low relative humidity causes desiccation, shrinkage and cracking, while high relative humidity can cause warping and swelling. Heat fluctuations induce similar expansion and contraction. These problems are particularly acute with thin ivory objects, such as miniatures.
Some darkening or "patina" is the result of the natural aging process of ivory's organic constituents. Because it is porous, ivory is also susceptible to staining. It darkens in contact with the skin or oils and can be stained by corroding metals or other colored materials.
Many liquids, including water and cleaning solutions, are destructive to ivory and should be avoided.
Handling
Handle ivory with white cotton gloves. If these are not available, wash your hands first with soap and water to remove hand oils and dirt.
Storage conditions
The best protection for an ivory object is a carefully controlled environment, both in terms of telative humidity (RH) and temperature. Ideal conditions are 45-55 % RH and approximately 70° F (21.11°C), with low light levels, at approximately 5 footcandles. Conditions should be kept constant; at the least, extreme conditions or rapid fluctuations should be avoided.
Keeping the object in a tightly closed display case or storage drawer provides a considerable degree of buffering against sudden changes in temperature and relative humidity and is protection against dust and dirt. Dark storage also eliminates damage due to light. However, as a note of caution, avoid sunlit or spotlit display areas, closed cases where heat can build up from interior light bulbs, proximity to ventilation or heating ducts, the tops of appliances, exterior walls or cold windows. Storage drawers and shelves should be lined with a chemically stable cushioning material such as polyethylene or polypropylene sheeting. Avoid using rubber-based materials for storage or packing as these can produce unnatural yellowing of ivory.
