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.
↑Weight conversions from USDA National Nutrient Database (NND), except for apple, for which a sample was grated, measured to 237 mL and weighed 10 times, and answers averaged. Original recipe text and ingredient order preserved. Brown sugar and raisins not specified as packed or unpacked, packed presumed. Large egg presumed. All conversions performed on U.S. customary (i.e. cups, teaspoons, etc.). USDA NND uses a cup size of about 237 mL, while the milliliter entries are using the FDA cup size of 240 mL.