One of the reasons I got a rice cooker was to be able to cook oatmeal overnight and have it ready to go in the morning. I have been trying to get away from those little Quaker Oats packages that are full of additives and other junk.
I tried quick-oats in the rice cooker once, only because that's all I had on hand. As I suspected, they came out a little mushy. Regular oats work much better. I'd love to be able to use milk, but letting milk sit in the machine all night doesn't seem wise.
All I do is follow the directions on the oatmeal package, set the timer to be ready for whatever time in the morning, add a few things like brown sugar, cinnamon, and a pinch of salt. The machine leaked a little the first time, so I add just a little less water than the recipe calls for.
One thing I found didn't work so well was adding raisins the night before. They come out really soft, almost mushy - I didn't like the texture. So now I add them after it's finished cooking along with some nuts - walnuts, pecans or almonds.
Sometimes the oatmeal is a little thick since I use less water, so I'll add some milk to get it the right consistency - at least this way, I can still have a little milk in the oatmeal.
I have also tried steel-cut oats with great success. I tried them overnight and the machine did leak a little through the top vent. So, I tried them in the morning with a little less water so that I could keep and eye on them and see if it leaked with less water. This time it leaked even more! I have to figure that letting them sit overnight allows the oats to soak up some of the water, resulting in less spill-over. Now I only do them overnight, adding just a tad less water than the recipe calls for and add a bit of milk the next morning to get the right consistency.
It's so nice to have a nice pot of oatmeal ready to go in the mornings without all the additives and without taking any extra time to prepare it.