In a medium mixing bowl, mix together flour, oats, salt, cinnamon, baking powder, and baking soda until well combined.
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.
Fold the dry ingredients into the wet ingredients until just combined. Add the raisins and walnuts to the batter and gently fold in.
Measure out approximately a tablespoon of dough and gently roll it into a ball. Place the dough on a nonstick, or parchment-lined cookie sheet, spaced out about 2-3 inches apart.
Bake for 6-7 minutes, until edges are set and lightly brown, The center of the cookies should still be a bit soft and jiggly.
Remove from the oven and cool for 2-3 minutes on the cookie sheet before transfering the cookies to a cooling rack to cool completely.
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Notes
Storing: Store Oatmeal Cookies in an airtight container for up to 5 days at room temperature. Add a slice of white bread to the container to help keep the cookies soft. Preparing in Advance: To prepare in advance, you can make the dough up to 2 days before baking and refrigerate it until baking. Alternatively, you can roll the dough into 1-inch balls and freeze it 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 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.