Preheat your oven to 350°F (175°C) and line a muffin tin with silicone liners or parchment paper.
Place the rolled oats into a high-speed blender and pulse until they reach a fine, flour-like consistency.
In a large mixing bowl, whisk together the Greek yogurt, liquid egg whites, and vanilla extract until smooth.
Add the oat flour, vanilla protein powder, baking powder, sea salt, and half of the ground cinnamon to the wet ingredients, stirring until just combined.
Gently fold the diced apples into the batter using a spatula.
In a small separate bowl, combine the almond flour, the remaining cinnamon, and the maple syrup to create a crumbly streusel topping.
Distribute the muffin batter evenly into the prepared muffin tin and sprinkle the streusel mixture over the top of each.
Bake for 18 to 22 minutes, or until a toothpick inserted into the center comes out clean and the tops are golden brown.
Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.