I've never added cinnamon or brown sugar to an oatmeal stout, but I've used them in a pumpkin spice ale that turned out nice.
One thing about spices (the cinnamon) to keep in mind: a little touch is nice, but it's REALLY easy to overdo it. I wouldn't use more than a couple or three sticks; once I had a pumpkin spice ale that was so full of cinammon and all-spice that I almost puked
The brown sugar (say a cup or so) I think will add some body to the beer and increase the ABV a bit, but I don't know for sure about taste. Actually, I don't know that much about brown sugar in beer in general