Here's a Quaker Oats recipe for a type of cookie/cake that works with cooked oatmeal; they recommend the recipe for either instant or regular oatmeal. I dislike instant oatmeal, but it has been pre-cooked, so this recipe works well with cooked oatmeal.
2-3/4 C old fashioned oatmeal, uncooked
2/3 C brown sugar [I use much less, perhaps 1/4 C, or maple syrup, a TBSP for flavor]
3/4 C raisins and/or dried cranberries (substitute other dried fruits)
1 apple, peeled and sliced (substitute other fresh fruits - optional)
1/4 to 1/3 C nuts, chopped (walnuts, almonds, sunflower seeds, hazelnuts..., or even better a mix of all - optional)
1 tsp ground cinnamon
1/2 tsp salt [You may not need any if the oatmeal is salted.]
3-1/3 C milk [cut back the milk if the oatmeal is cooked -- 2 cups may be enough]
2 eggs, lightly beaten
1 Tbsp vanilla extract
Preheat your oven to 180°C (360°F). Lightly oil an 13 by 9-inch baking dish.
In a large bowl, combine the sugar, raisins, chopped nuts, cinnamon and salt. Mix well. In a medium bowl, combine the oatmeal, milk, eggs and vanilla. Mix well. Add this to the larger bowl and mix until well blended.
Pour the mixture into the baking dish, and make sure the solids are more or less evenly spread out. Arrange the apple slices over the surface.
Put in the oven to bake for 55 to 60 minutes, until the center is set and firm to the touch. Let cool slightly.
You might want to check for doneness after 45 minutes or so. You can hold left over oatmeal for two or three days to get enough to make these cookies.
I like this recipe because of all the substitutions that are possible -- a rent a kid came up with a chocolate chip version of this cookie that was delicious.
Another idea: I sometimes make a pancake from left over oatmeal and sautee it lightly until a nice crust forms on both sides. Serve with maple syrup and butter.