Old fashioned oats, sometimes referred to as rolled oats, are the best thing to use in this oatmeal raisin cookies recipe. That's not to say that instant oats can't be used. They can...but the cookies will have a different texture, and will be softer. Instant oats cook faster, so the bake time for the cookies may need to be reduced. Substituting instant oats involves a lot of guesswork, so eliminate all of that by using old fashioned oats.