Olive oil instead of butter
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Remember when i talked about the liquid water content in cookies? Using a liquid oil will also cause a lot of spread. The flavor change wouldn't be to my liking. And you can't switch oil for butter in all cakes. Any cake made with the creaming method will not work well with oil because creaming is also a leavening action. You create air holes in the butter by dragging the sugar crystals through the butter.
I do sometimes make an olive oil substitution for butter in biscuits. I use less oil because it's thin and it doesn't create the flakiness. It's not bad, but it's a rather different sort of biscuit.