Preheat oven to 350 degrees F.
Using a stand or hand-held mixer, cream the butter, eggs, and sugars until well combined. Omit powdered sugar for a chewier cookie. Add vanilla and mix until incorporated.
Sift together baking soda, salt, cinnamon, and flour. Add to butter mixture until completely incorporated.
Add 3 cups of oatmeal to the mixture. If it is too gooey to work with, add more until it can be rolled into balls. Add the nuts or raisins if using.
Place small balls of dough on a slightly greased cookie sheet and flatten slightly before baking. Sprinkle with decorations, if desired. Make sure to leave enough space between cookies as they spread quite a bit. If you omit the powdered sugar, leave even more space, perhaps only 6 cookies per cookie sheet.
Bake for 8-10 minutes. They will spread and be flat and chewy. If you use the powdered sugar, they will be flat and crispy. Adding more oatmeal and raisins/nuts will make them thicker and chewier.
Remove from cookie sheet to a cooling rack immediately or they will stick.