Muffins are one of many traditional breakfast quickbreads; unlike biscuits, muffins usually contain some form of fruit or other garnish. These muffins are especially healthy because they are made with whole wheatflour and oats. See the notes at the bottom of the recipe for ideas about how to customize this muffin recipe to your own tastes; the basic proportions should work for any flavor of oat muffin.
Overmixing will lead to worming, which is caused by gluten development. You will see this as vertical tunnels running through your muffins, much like a worm might leave. It will also lead to tough and chewy muffins.