The difference between steel cut, rolled, and instant oats
Steel cut oats— The whole oat is cut into pieces which look like grains of rice. These oats take the longest to cook and typically aren’t used in baking recipes.
Old-fashioned/rolled oats— The whole oat is pressed flat during processing. These oats cook much faster than steel cut oats and are the most common oats used in baking recipes.
Quick-cooking/instant oats— These oats are precooked, dried, and pressed flat. They cook very quickly, but don’t retain much of their texture. They can be used in baking recipes, but the baking time will need to be adjusted.