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)
cooking.stackexchange.com/users/207/Recep
If you're making something like muffins, then in my experience the water content is a non-issue.
Yes, there's some water.
But muffin batter has such a broad range of tolerance that as long as you're basically familiar with the target consistency it'll be fine.
That said, the amount of oil or butter that goes into a batch of muffins is not really enough to impart much butter flavor.
cooking.stackexchange.com/users/557/Pointy
You'd be better off using clarified butter than melted butter, as butter is about 15% water, and could affect the texture if not compensated for properly.
cooking.stackexchange.com/users/67/Joe
If I am substituting butter for oil, I like to do a mixture of 1/2 butter and half shortening, this allows the richness of the butter as well as keeping the consistency of the recipe.
I also add clear butter extract (flavoring) to the recipe.
I use about 1/2 tsp butter extract per 1/2c of shortening and cream the butter/shortening mixture with eggs, sugar and vanilla first before I add any other ingredients.
I also substitute shortening for butter sometimes, only I also add 1tbs of water per 1/2c shortening along with the extract.
Hope this helps!
cooking.stackexchange.com/users/40286/Melissa Burgess
two options spring to mind:1) consider either adding butter flavoring (I have only seen the imitation version int eh extract flavoring options at the stores) 2) or just adding a little butter to the batter (substituting part or even just throwing in the leftovers of a stick)
cooking.stackexchange.com/users/19130/jenny
When a recipe (like for muffins) calls for vegetable oil, does it matter if I substitute the oil for butter because I want that buttery taste in the muffins?  Do I need to compensate for the difference by adding/subtracting the amount of butter added?
Substituting butter for oil: Does it matter for baked goods?
cooking.stackexchange.com
<baking><substitutions><oil><butter>
cooking.stackexchange.com/questions/2323
cooking.stackexchange.com/users/689/zachary
Butter is over 15% water, so you should add more butter, clarified butter or a mixture of butter and shortening to ensure a similar consistancy. Use a mixture of butter and shortening or butter with a little liquid.
You will need to add more butter than oil to compensate for the difference in liquid. You can do a mixture of butter and shortening to keep the same consistency.