Substitute Olive Oil for Butter: Olive oil offers so much more nutrient value with no downside, unlike the alternatives.
You can substitute olive oil for butter, margarine or other oils in almost any recipe. The basic rule is to use an equal amount of olive oil if the recipe calls for vegetable oil. The variety of olive and harvest time are major factors affecting the flavour of olive oil. If you want to replace butter with olive oil, to use in baking or on delicately flavoured foods, you don’t want an olive oil that is described as “grassy, peppery, or pungent”. An olive oil described as “smooth, mellow, and buttery” is what you’re looking for and these fit the bill.
(from Amazing Olive Oil.com)