Preheat your oven to 350°F (175°C) and position racks in the upper and lower thirds. Line 2–3 baking sheets with parchment paper or silicone mats, and ensure butter and eggs are at room temperature.
In a medium bowl, whisk together the flour, baking soda, cinnamon, and salt for about 30 seconds until evenly combined. Stir in the oats, then set the bowl aside.
Beat the softened butter with both sugars on medium speed for 3–4 minutes until the mixture is light, fluffy, and pale. Scrape down the sides of the bowl as needed.
Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract until fully combined and smooth.
With the mixer on low, add the flour-oat mixture in three additions, mixing just until no streaks of flour remain. Do not overmix.
If using raisins, chocolate chips, or nuts, gently fold them into the dough with a spatula until evenly distributed.
Scoop dough into 1.5-tablespoon balls, roll between your palms, and place 2 inches apart on the prepared baking sheets (about 12 per sheet).
Use the bottom of a glass or your palm to gently flatten each dough ball to about ½ inch (1.25cm) thick for more even baking and a rounder shape.
Bake for 11–13 minutes, rotating sheets halfway through, until edges are golden brown and centers look slightly underdone. They will firm up as they cool.
Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely. Store in an airtight container at room temperature for up to 10 days.