Mix flour, oats, sugar, cocoa powder, chia seeds, baking powder, and salt together in a large bowl. Add 3 cups almond milk and coconut oil; stir with a wooden spoon until batter is just combined. Cover and refrigerate, 8 hours to overnight.
Preheat a Belgian waffle iron according to manufacturer's instructions.
Stir 1 1/2 to 2 cups almond milk into the batter to thin it to the consistency of cake batter.
Scoop 1/2 cup batter onto the preheated iron and spread to the edges. Close iron and cook until iron stops steaming, about 5 minutes. Repeat with remaining batter.