Heat oven to 375°F and lightly grease a 9x13 inch baking pan.
Combine oats, brown sugar, baking powder, salt and cinnamon in a large bowl. Add eggs, milk, melted butter and vanilla and mix very well.
Spread in prepared pan. Bake for 40 minutes, or until set. Let cool for 5 minutes, then slice into bars to serve.
To prep the oatmeal now and bake later:Make the oatmeal up to adding the mixture to the baking dish. Cover and refrigerate up to overnight.Bake in the morning, uncovered, as directed in the recipe. You'll need to add around 10 minutes to the baking time because the oatmeal is very cold from being in the fridge all night.