The trick is to grind the oatmeal into a powder before adding it to the smoothie. You can usually do this right in your blender. Just measure out around a quarter cup or so of oatmeal (per smoothie), give it a whir in the blender, and then add the rest of your smoothie ingredients.
The result is a thicker smoothie with an added nutritional boost! The oatmeal blends right in with the milk, fruits, and whatever else you've added. The starchy oatmeal absorbs some of the liquid, giving the smoothie more body and substance. This is especially good if you make your smoothie the night before or end up letting it sit for a little while before drinking it.
If the blender doesn't seem to be grinding the oatmeal finely enough for you, try using a spice grinder or food processor. You can grind several cups at once, store it in a jar, and then scoop out the amount you want every day. We're also curious to try toasting the oats beforehand to see if that gives us a richer flavor!
Do you ever put oatmeal in your smoothie?