Preheat oven to 425 degrees. Line muffin pan with paper baking cups.
In large bowl, combine oats and buttermilk. Let stand for 30 minutes. On waxed paper, combine flour, baking powder, baking soda, cinnamon, salt, and nuts.
Add oil, egg, brown sugar, and vanilla to oat mixture and combine. Stir in flour mixture until just combined. Divide batter evenly among muffin cups, filling about two-thirds full. Bake 12-16 minutes, or until a wooden pick inserted into center of muffin comes out clean. Cool on rack prior to freezing. To freeze, place muffins in individual plastic sandwich bags, and place into large plastic zipper bag. Makes 12 muffins.