Preheat and prep: Heat your oven to 375°F (190°C).
Line a 12-cup muffin tin with paper liners or lightly grease it.
Combine the dry ingredients: In a large bowl, whisk together 1 cup rolled oats, 1 cup whole wheat flour, 1 1/2 teaspoons baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt, and 1 teaspoon cinnamon.
Mix the wet ingredients: In a separate bowl, whisk 1/2 cup Greek yogurt, 1/2 cup mashed ripe banana (or applesauce), 2 large eggs, 1/3 cup honey or maple syrup, 1/2 cup milk, 1 teaspoon vanilla, and 2 tablespoons oil.
Bring it together: Pour the wet mixture into the dry ingredients. Stir gently with a spatula until just combined. The batter should be thick but scoopable. Do not overmix.
Fold in add-ins: Stir in up to 1 cup total of your choice of fruits, nuts, or chocolate.
If using berries, toss them in a teaspoon of flour first to help prevent sinking.
Fill the pan: Divide the batter evenly among the 12 cups. For a nice top, sprinkle a few extra oats on each muffin.
Bake: Bake for 16–20 minutes, or until the tops are set and a toothpick comes out clean or with a few moist crumbs.
Cool: Let muffins cool in the pan for 5 minutes, then move to a wire rack. This helps them set and prevents soggy bottoms.