Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and next 7 ingredients (through salt) in a large bowl, stirring with a whisk. Stir in oats, dates, raisins, and cranberries. Make a well in center of mixture. Combine buttermilk, oil, vanilla, and egg; add to flour mixture, stirring just until moist. Stir in boiling water. Let batter stand 15 minutes.
Preheat oven to 375°.
Spoon batter into 12 muffin cups coated with cooking spray. Bake at 375° for 20 minutes or until muffins spring back when touched lightly in center. Remove muffins from pans immediately; place on a wire rack.