Preheat the oven to 425 degrees. Spray a baking sheet generously with cooking spray and set aside. Sift the flour into a large mixing bowl. Create a well in the center of the flour and pour in the buttermilk followed by the oil. Add the salt. Mix all ingredients together just until all the flour is moistened. Do not overmix. Turn the dough out onto a very lightly floured surface and knead 4 or 5 times – no more. Pinch off portions of dough a little larger than a golf ball. Roll the dough into a ball, then press to flatten into a disk. Place the biscuits on the prepared baking sheet with the edges touching. Place the baking sheet in the upper third of the oven and bake for approximately 15 minutes. If tops are not quite brown near the end of the cooking time, turn on the broiler briefly to finish browning. While the biscuits are cooking, melt the butter over low heat. Remove biscuits from the oven and brush with melted butter.