Can you post a picture?
Getting oil out is probably not going to happen. I think I would use some deglazer on it first. Then dye with Fiebings Professional Oil dye. Then resealed. They make a non oil dye, but I never use it. Professional Oil dye is best IMO. But it is also twice as expensive as the cheaper dye. Cost me $80 per gallon.
If you apply products (like oils) to leather, they generally soften the leather. Softening a holster is not good IMO. It can render the holster useless by getting too soft. I recommend using Renaissance Wax and you can get it at Midway USA.
By the time you buy what is needed you might have paid a significant amount of a new holster. Is there a shoe repair near you? They may be able to dye for you, but ask if they have the Professional Oil dye. If you want to send it to me to try, give me a call to discuss.