Prepare the Baking Equipment: Preheat your oven to 350°F (175°C) and prepare two baking sheets by lining them with parchment paper or silicone baking mats. This step ensures your cookies won’t stick to the pan and makes cleanup easier.
Cream the Butter and Sugars: Using a large mixing bowl, combine the softened butter with brown and granulated sugars. Blend the ingredients together thoroughly using either a hand mixer or a stand mixer until they form a creamy consistency. Mix for 2–3 minutes until the mixture becomes light, fluffy, and pale. This process incorporates air, giving the cookies a tender texture.
Add Wet Ingredients: Incorporate the eggs individually, mixing each one in thoroughly before adding the next. Once all eggs are combined, blend in the vanilla extract. Make sure to scrape down the sides of the bowl to incorporate all ingredients fully. The wet mixture should look smooth and creamy.
Mix the Dry Ingredients: In a separate medium-sized bowl, whisk together the flour, baking soda, ground cinnamon, and salt (if using). Slowly incorporate the dry ingredient mixture into the wet components, using a mixer on its lowest setting. Continue until the ingredients are blended, taking care not to overmix.
Fold in the Oats and Raisins: Using a spatula or wooden spoon, gently fold in the old-fashioned oats and raisins. Ensure the add-ins are evenly distributed throughout the dough for a consistent flavor and texture.
Portion the Dough: Using a cookie scoop or spoon, portion it into tablespoon-sized balls, and arrange them on the prepared baking sheets. Leave approximately 2 inches of space between each ball to accommodate the cookies' expansion during baking.
Bake the Cookies: Place the baking sheets in the oven and bake for 8–10 minutes until the edges are golden brown and the centers appear set but soft.
Cool the Cookies: Leave the baked cookies on the tray for a couple of minutes to set before moving them to a cooling rack to finish cooling. Enjoy them either still warm or after they've reached room temperature.