In a large bowl, mix together flour, oats, salt, cinnamon, baking powder, and baking soda.
In a large mixing bowl or stand mixer, cream butter with brown and granulated sugar together until fluffy and light in color--about 3-5 minutes. Add the eggs and vanilla and beat until well combined.
Stir the dry ingredients in the wet ingredients until just combined.
Add the raisins and walnuts, if using, to the batter and gently fold in.
Refrigerate dough for at least 30 minutes.
Preheat oven to 375 degrees F.
Measure out approximately a tablespoon of dough and roll into a ball. Place onto a parchment-lined, or non-stick cookie sheet, spaced out about 2-3 inches apart.
Bake for 6-7 minutes, until edges are lightly browning and centers are still a bit jiggly. Cool on cookie sheet for a few minutes and then transfer to a cooling rack to cool completely.
Storing: Store Oatmeal Cookies in an airtight container for up to 3 days at room temperature.Preparing in Advance: To prepare in advance, you can make the dough up to 2 days before baking and refrigerate until baking. Alternatively, you can roll the dough into 1 inch balls and freeze on a cookie sheet until frozen solid. Once frozen, pop into a freezer-safe bag. To bake from frozen, bake at 375 degrees for 7-9 minutes.Optional Flavors:
Oatmeal Chocolate Chip Cookies: Omit the raisins, walnuts, and cinnamon and add in 2 cups of semi-sweet chocolate chips to the cookie dough batter.
Cranberry White Chocolate Oatmeal Cookies: Omit cinnamon, walnuts, and raisins and mix in 1 ½ cups dried cranberries, 1 cup white chocolate morsels, and 1 teaspoon orange zest.
Coconut Oatmeal Chocolate Chip Cookies: Omit the raisins, walnuts, and cinnamon and mix in 1 cup semi-sweet chocolate chips, 1 cup shredded coconut, and 1 cup chopped almonds.