I find that the old-fashioned rolled oats are the best to use for overnight oats. They will soften up more quickly but still have plenty of texture, so the mixture isn't too soft and mushy. Instant oats have little to no texture after they have soaked, so they would not be the best choice for this recipe. Steel-cut oats require a much longer soak time and will need extra liquid, so also not the best choice for this recipe.