Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease it lightly.

In a large bowl, whisk together the flour, sugar, salt, and baking powder until evenly distributed.

In a separate bowl, whisk the vegetable oil, egg, vanilla, and milk until smooth.

Pour the wet mixture into the dry ingredients and fold gently with a spatula until you no longer see dry streaks. A few lumps are okay.

Fold the blueberries into the batter carefully so they stay whole.

In a small bowl, stir the brown sugar, rolled oats, and melted butter to form the crumble topping. It should be clumpy.

Fill each muffin cup about two-thirds full with batter, then sprinkle the crumble evenly over each filled cup.

Bake for 20–25 minutes, rotating the pan halfway through if your oven bakes unevenly. Muffins are done when the tops are golden and a toothpick inserted in the center comes out clean or with a few moist crumbs.

Cool in the pan for 5 minutes, then transfer to a wire rack to finish cooling. Serve slightly warm or at room temperature.