Preheat the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease the muffin cups.
In a large bowl, combine the rolled oats, whole wheat flour, brown sugar, baking powder, baking soda, salt, and ground cinnamon. Stir well to mix evenly.
In a separate medium bowl, whisk together the Greek yogurt, eggs, vegetable oil, vanilla extract, and milk until smooth.
Pour the wet ingredients into the dry ingredients. Stir gently until just combined. Do not overmix; the batter will be thick.
Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Bake in the preheated oven for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and allow them to cool in the pan for 5 minutes.
Transfer the muffins to a wire rack to cool completely before serving.