Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats.
In a large mixing bowl, beat the softened butter, light brown sugar, and granulated sugar together until light and creamy, about 2–3 minutes.
Add the eggs one at a time, mixing well after each addition. Beat in the vanilla extract until smooth.
In a separate bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, and salt.
Add the dry ingredients to the butter mixture and mix on low speed just until combined and no streaks of flour remain.
Stir in the rolled oats with a spatula, then fold in your choice of mix-ins (raisins, chocolate chips, nuts, and/or shredded coconut) until evenly distributed.
Scoop the dough into heaping tablespoonfuls (or use a small cookie scoop) and place them on the prepared baking sheets, spacing them about 2 inches apart.
Bake for 10–12 minutes, or until the edges are lightly golden and the centers still look slightly soft.
Remove the baking sheets from the oven and let the cookies cool on the pan for 5 minutes, then transfer them to a wire rack to cool completely.
Serve warm or at room temperature. Store leftover cookies in an airtight container at room temperature for up to 5 days.