If doing every oatmeal cup the same, add the mix-in to the batter and combine. I tend to do a few different kinds, so I don't mix it in with the batter. Fill the muffin cups to the top with the batter.
If doing different type of oatmeal cups: fill muffin tips about 1/2 way. Add a little bit of your mix-in as desired (blueberries & chocolate chips, etc). Fill the rest of the muffin tin to the top.
Add a few more of your mix-in, if desired (I always do!)