Substituting butter for oil: Does it matter for baked goods?
Butter is at most 82% oil and the rest is water. So, you're going to need to add more butter for the same amount of oil. Besides, you need to adjust the liquid content accordingly since you are going to have more water than you planned.

If you are going to use the same amount of oil, then it will not make any difference other than the taste. Just make sure they are of same state (i.e. melted butter instead of vegetable oil or vice versa)