Sprinkle the yeast over the water in a large mixing bowl; let stand until foamy, about 5 minutes. Add the buttermilk, oil, sugar and flour and whisk until smooth. Cover the bowl with plastic wrap and refrigerate overnight.
Preheat a waffle iron. Whisk the oats, egg whites, baking soda, and salt into the waffle batter until smooth. Lightly mist the hot waffle iron with cooking spray. Add about 1/3 cup of batter to each section, using the back of a spoon to spread batter to the edges. Cook until the waffles are crisp and golden brown, 5 to 7 minutes. Repeat with the remaining batter. Serve with maple syrup.