Preheat oven to 400°F (200°C). Grease or line 12 standard muffin cups.
Whisk together the flour, baking powder, and salt. Stir in the oats. Set aside.
Using an electric mixer on medium speed, beat the butter and brown sugar until light and fluffy. Add the eggs, one at a time, mixing well after each addition. Mix in the vanilla.
Reduce mixer speed to low. Add the flour mixture in 3 portions, alternating 2 portions of milk. Mix just until combined. Stir in the nuts and seeds.
Fill each muffin cup about 3/4 full using about 3 tablespoons of batter per cup.
Bake 18 to 22 minutes, or until the muffins are browned and a pick inserted into the center comes out clean.
Cool the muffins in the pan for about 10 minutes. Then transfer the muffins to a wire rack to cool completely.