Heat the oven to 350 degrees F and line 2 standard-sized muffin pans with paper liners. In a large bowl, stir together the flour, oats, brown sugar, flaxseed, baking soda, baking powder and cinnamon.
Add the eggs, oil, buttermilk and 3/4 cup water. Mix until the dry and wet ingredients are just combined, and then fold in the blueberries. Spoon the batter into the muffins cups, filling each cup right up to the top, and bake the muffins for 20 to 25 minutes, until a toothpick inserted into the center comes out clean. Cool in the pans for 5 minutes, then turn out onto a rack and cool completely before serving.