Grease a 9x13" baking dish with cooking spray. Set aside.
Whisk together all dry ingredients in a large mixing bowl, except raisins and nuts.
Whisk together all wet ingredients in a separate bowl.
Combine wet and dry ingredients thoroughly, then stir in nuts and raisins.
Pour into the baking dish and bake for 30 minutes.
Cool for 10-15 minutes before slicing into 12 squares. Serve with yogurt, whipped cream, maple syrup, or your favorite topping.
Notes
To make ahead, you can mix the baked oatmeal batter and pour it into the baking dish the night before. Simply cover and refrigerate overnight and bake it in the morning.
For sweeter baked oatmeal, add a small amount more of maple syrup and drizzle more on top after baking.