Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch (or similar size) baking dish.
Step 2: In a large bowl, whisk together the eggs, oil, maple syrup (or honey), orange juice, milk, and vanilla extract until well combined.
Step 3: Add the gluten-free rolled oats, orange zest, baking powder, ground cinnamon, and sea salt to the wet ingredients. Stir until everything is just moistened. Be careful not to overmix.
Step 4: Gently fold in the fresh or frozen cranberries into the oat mixture.
Step 5: Pour the mixture evenly into the prepared baking dish. Spread it out with a spoon or spatula if needed.
Step 6: Bake for 35-40 minutes, or until the oatmeal is set, golden brown around the edges, and the top springs back when lightly touched.
Step 7: While the oatmeal bakes, prepare the optional glaze: In a small bowl, whisk together the confectioners sugar and orange juice until smooth. If too thick, add a tiny bit more orange juice; if too thin, add a bit more sugar.
Step 8: Remove the baked oatmeal from the oven and let it cool for a few minutes before serving. Drizzle with the orange glaze, if desired, before serving warm.