Position the oven rack in the center slot and set the oven to preheat at 350°F (176°C) while you make the cookie dough. Line a baking sheet with parchment paper or a silicone baking mat.
Begin by mixing rolled oats, all-purpose flour, raisins, ground cinnamon, salt, and baking soda in a bowl and set aside. (Include chocolate chips or nuts if desired.)
In a separate large bowl, use a spatula or wooden spoon to combine the softened butter with brown and granulated sugar until the sugar begins to dissolve. Add the sourdough starter, egg, and vanilla extract, and stir until the mixture is smooth.
Combine the dry ingredients with the wet ones and gently fold them until just mixed. (You can refrigerate the dough for up to 2 days or proceed with baking immediately.)
Use a cookie scoop to divide the dough into 12 portions on the baking sheet, ensuring approximately 1 ½ inches of space between each. Each portion should be around 2 tablespoons, roughly the size of a golf ball.
Place the baking sheet in the oven and bake for 10-12 minutes or until the cookies turn a light golden brown. Take them out and let the cookies rest on the tray for 10 minutes before moving them to a wire rack to cool before serving.