Preheat the oven to 350 F (postpone this step if you are planning to refrigerate your dough for a couple of days). Line a baking sheet with parchment paper if you have it.
Combine the flour, baking soda, cinnamon, and salt in a medium-size bowl.
Blend the butter and both sugars together in a large bowl with a hand mixer (or a stand mixer) until well blended. Beat in the egg, then beat in the vanilla. Add the flour mixture gradually, mixing at low speed until each batch is incorporated. Mix in the oats and the raisins. If desired, refrigerate the dough for up to 4 days.
Bring the dough to room temp if you've refrigerated it. Use a 1/4 cup measuring cup (or cookie scoop or ice cream scoop) to measure out balls of the dough, which should be placed on an ungreased cookie sheet with at least 3 inches in between each ball (you should bake 6 cookies per classic-sized cookie sheet). Use the bottom of a glass to flatten each ball.
Bake the cookies for about 11 to 16 minutes (see Note! This is important!) until they are golden brown, but still have nice give in the middle. Let them sit on the baking sheet for a couple of minutes before transferring them to a cooling rack.
Notes
If your cookie dough is at room temperature, whether you’ve just made it or you’ve let it return to room temperature before baking, you must start checking the cookies at 11 minutes. They tend to go from almost done to definitely done very quickly. The longer baking time is if you are baking them straight from the fridge.