Preheat oven to 375°F. Spray muffin pan with nonstick spray.
In a large mixing bowl, combine flour, oats, sugar, poppy seeds, baking powder, baking soda and salt. Mix well with a wire whisk to combine.
In a separate bowl, combine milk, oil, egg, lemon juice, clementine juice, lemon zest and clementine zest. Whisk well to combine.
Next, add the wet ingredients to the dry ingredients and whisk until fully mixed. Set aside to rest while you make the streusel.
To make the streusel, combine oats, sugar, cold butter, lemon zest and clementine zest. Mix well with a fork until the mixture resembles a coarse meal. Set aside.
Evenly distribute batter into 5 muffin cups in the prepared pan. Evenly sprinkle the streusel topping on the muffins.
Bake for 15–20 minutes, or until golden brown and a toothpick inserted in the center of a muffin comes out clean.
Allow muffins to cool for 5 minutes before removing from the pan. Cool completely on wire rack.
