In a large bowl, combine oats and buttermilk and let stand 1 hour or overnight.
Preheat oven to 400 degrees. Butter 12 ½-cup muffin tins.
Mix egg, sugar, and butter into oat mixture, stirring just until combined. In another large bowl, sift together flour, salt, baking powder, and baking soda, then add to oat mixture, stirring just until combined. Fold in raisins. Divide batter evenly among prepared muffin tins. Sprinkle the tops with raw sugar. Bake muffins about 20 minutes in the middle of the oven, until golden and a wooden pick inserted in the center comes out clean.