butter, lots of butter (I use about 1/4+ cup)
When storing away the left-over oatmeal, shape it into a log. (Alternate idea: grease a glass or other cylindrical container and fill it with oatmeal before you refrigerate it. Theoretically, it should slide right out when you're ready to cook, and you can slice it into nice, neat round slices.)
Melt butter in a frying pan. Slice the log of oatmeal into about 1/2 inch slices and place in pan. Cook until heated through and each side is golden brown, a few minutes per side. Serve like pancakes, with maple syrup on top.