Preheat the oven to 190 degrees C. Lightly coat a 12-hole muffin tray with cooking spray; each hole should be about 6cm diameter. Evenly spread out the oats on a baking tray and bake for about 10 minutes or until golden and crisp, stirring occasionally. Transfer to a large bowl and allow to cool to room temperature.
Add the plain and wholemeal flours, wheatgerm, 1/3 cup (75g) of the sugar, the baking powder and bicarb soda to the oats, stirring to combine. Whisk together the buttermilk, oil, egg and orange zest in a small bowl until blended.
Make a well in the centre of the dry ingredients and pour in the buttermilk mixture. Stir just until the dry ingredients are moistened. Fold in the cranberries.
Divide the mixture evenly among the muffin tins. Sprinkle the top of each muffin with 1/4 tsp sugar. Bake for about 30 minutes or until golden brown and a skewer inserted in the centre of a muffin comes out clean. Remove the muffins from the tray to a wire rack to cool.