Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease it lightly to prevent sticking.

In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until well combined. This ensures even distribution of the leavening agents.

In another bowl, combine the buttermilk, vegetable oil, and egg. Whisk until smooth. This mixture adds moisture and richness to the muffins.

Pour the wet ingredients into the dry ingredients and gently fold with a spatula until just combined. Be careful not to overmix, as this can lead to dense muffins.

Gently fold in the diced peaches, ensuring they are evenly distributed throughout the batter.

In a medium bowl, combine the flour, brown sugar, and cinnamon. Mix well to combine.

Add the cold, cubed butter to the mixture. Using your fingers or a pastry cutter, mix until the mixture resembles coarse crumbs. This will create a crunchy topping.

Stir in the rolled oats for added texture.

Spoon the muffin batter into the prepared muffin tin, filling each cup about 2/3 full.

Sprinkle the crumble topping generously over each muffin.

Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean. This ensures the muffins are cooked through.

Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.