Preheat oven to 375 degrees.
Make muffin batter: Line your muffin tray with tins or spray with cooking spray. Whisk together the all-purpose flour, baking powder, salt, and oats. Add in the strawberries, raspberries, and blueberries. Mix until everything is well coated. Set aside.
With a hand mixer, cream together the butter and sugar until light and airy. Mix in the eggs one at a time, making sure it is fully incorporated before adding the next one. Slowly add the half and half and mix until all of the ingredients are combined.
Fold the dry ingredients into the wet 1/2 cup at a time until everything is incorporated. Fill the muffin tin with batter and set aside.
Make topping: Combine the cinnamon, nutmeg, oats, flour, and brown sugar in a bowl. Cut in the butter until small clumps of butter remain.
Assemble muffins: Top each muffin with 2 tablespoons of oat topping. Bake for 35 minutes. Cool completely before serving.