Preheat a waffle iron according to manufacturer's instructions.
Stir milk, zucchini, eggs, and butter in a bowl until evenly blended. Combine flour, oats, baking powder, brown sugar, salt, and cinnamon in another bowl; gradually stir flour mixture into zucchini mixture.
Ladle batter into preheated waffle iron. Cook until waffles are golden and crisp, about 5 minutes.
