Let’s look at oats for a moment.
Whole oats, from usda.gov
Here, we have whole oats–the hull (where the bran comes from) and the interior (the groat, where rolled oats come from) together. The hull of whole oats is very hard, so to make it easier to cook and eat outs, we usually remove it; the interior can then be steamed and rolled, producing a flattened grain that looks like the oats we generally encounter. The hull goes on to live in oat bran products.
When you see a recipe that just says rolled oats, it typically means old-fashioned rolled oats. But quick-cooking oats and instant oats are rolled, too, just progressively thinner. While you could hypothetically substitute any rolled oat for any other, overcooked instant oats turn into mush, and undercooked old-fashioned oats can taste raw.
But there’s another school of thought on how to deal with the hard bran coating, and that’s the steel-cut oat. These are whole oats that have been chopped into pieces, hull and all. Keeping the bran (more or less) intact gives these a bit more nutritional punch and a nice nutty flavor in a porridge, but don’t substitute them for regular oats in a recipe, unless you want to see people wince, look confused, and then check their teeth for cracked fillings.
But that said, the old fashioned rolled oat cookie has been a favorite for generations. Here’s a mention from the November 15, 1883 edition of the Nappanee (Indiana) News, although it’s a little light on the details:
— Oatmeal cookies combine many good qualities, and will be relished by children. Make them just like an ordinary sugar cooky, using two-thirds oatmeal and one-third wheat flour. — N.Y. Post.
Hrm. A 1913 version from Dishes & Beverages Of The Old South by Martha McCulloch-Williams is slightly more informative:
Oatmeal Cookies: (Mrs. T. G. Petre.) Beat together until creamy, one egg, half cup sugar, third cup butter, third teaspoonful soda mixed with one cup sifted pastry flour, half teaspoonful each of salt and cinnamon, then add one cup rolled oatmeal, half up each of shredded nuts and raisins. Mix well, drop on greased tin, and bake in a slow oven. Do not let the stiffness of the dough induce you to add milk or water.
From the box of A.D. from Lutz, Florida, by way of Pennsylvania in the 1940s, and originating in Ohio in the 1920s.
1 cup sugar
1/2 cup melted butter
pinch of salt
1 egg, beaten
1/4 teaspoon vanilla
2 cups rolled oats]
Cream 1 cup of sugar and 1/2 cup melted butter; pinch of salt, one egg beaten together, 1 teaspoon vanilla and 2 cups of rolled oats.
Drop from teaspoon on greased pan.