Hi Starr, the issue with substituting oil for melted butter (or similar non-dairy butter substitutes) is that butter consists of only 80% fat, and generally contains about 15% water, which will evaporate during baking. Oil, on the other hand is 100% fat. Substituting an equal amount of oil is likely to result in the cookies spreading and/or being too greasy. However, I think you could experiment with a smaller quantity of oil and get similar results, although the exact amount of oil to add to each recipe may require a bit of tweaking. This article suggest substituting about 3/4 to 7/8 the amount of oil as melted butter called for in a baked good like cookies. 

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