Butter is 80% water, so you can do a substitute, just do the math and either increase or decrease the liquid (depending for which you are subbing). If a recipe calls for 1 cup butter, you could do 13 tablespoons oil and 13 tablespoons water, for example. This, however, really only works in breads and/or quick breads. For cookies, which depend on the set-up and texture of better, I’d be a bit more leery about making the substitution.
I think you might have gotten a bit mixed up here. Butter contains between 80-85% butter fat, and only between 15-18 % water, and a small percentage of dairy solids (all this depends on the brand of butter, European style butters will have more butterfat and less water). You can substitute butter for oil provided you do one of two things:
1. Decrease the amount of liquid in the recipe to compensate for the amount of water in the butter (the amount of dairy solids won’t affect the recipe enough to be noticeable).
However, since it’s difficult to determine how much water butter actually contains unless you have a chemistry lab in your kitchen, you can:
2. Clarify the butter to remove all the water from it, then cool it down slightly before using it. If you are using clarified butter for quick breads and muffins, make sure you add it to the batter while it is still melted and warm (and make sure all your other ingredients are at room temp or slightly warmed—cold eggs and cold liquids will cause the butter to solidify which can affect how the batter emulsifies).
I have used the second method with excellent results. I have also used commercially made ghee (which is basically clarified butter with the dairy solids slightly browned), which can actually be quite tasty in a simple cookie or quick bread.