Steel-cut oats are the least processed of the two varieties because they are cut rather than steamed and rolled. Large steel blades are used to chop the groats into two or three pieces leaving steel cut oats with a texture that is chewier and coarser than most other forms of oats. Steel cut oats retain the hull which has most of the nutrition. They have a lower glycemic index than rolled oats which is important in some dieting circles. Steel-cut oats take much longer to cook than the rolled type because they are pretty much "right off the farm," but it is well worth the extra time and effort for their superior flavor. Whole grains such as oats and brown rice should be soaked for 12 to 24 hours before cooking. This will neutralize the phytic acid and break down the enzyme inhibitors before consumption so you will be able to obtain the full benefit of all the vitamins and minerals.
The three types of rolled oats are:
Old-fashioned – These oats have been steamed, dried, sliced and then flattened to produce the flat oatmeal shape.
Quick-cooking – The quick cooking oatmeal is steamed and goes through the the roller, flattened etc more than the the "old-fashioned" oatmeal.
Instant – The instant variety is the most processed of the three and is precooked