Remove butter from the refrigerator 30–60 minutes before starting so it reaches a soft-but-still-cool consistency. Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats.

Cream butter and sugars

In a large bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter, granulated sugar, and brown sugar on medium speed until light and fluffy, about 2–3 minutes. Scrape down the bowl as needed.

Add egg and vanilla

Add the egg and vanilla extract to the butter-sugar mixture and beat until fully incorporated, about 30–45 seconds.

Combine dry ingredients

In a separate medium bowl, whisk together the all-purpose flour, baking soda, and salt until evenly combined.

Mix dry into wet

With the mixer on low speed, add the flour mixture to the butter mixture in two additions, mixing just until combined after each addition. Avoid overmixing — stop as soon as the flour streaks disappear.

Fold in oats and mix-ins

Using a spatula, fold in the rolled oats, chocolate chips, and optional chopped nuts until evenly distributed throughout the dough.

Portion the dough

Use a 1 1/2 tablespoon cookie scoop (or spoon) to portion dough into roughly 1-inch-high mounds. Place mounds on the prepared baking sheets spaced about 2 inches apart to allow for spreading. For uniform cookies, you can roll each mound slightly between your palms to form rounds.

Baking and finishing

Bake

Bake cookies in the preheated 350°F (175°C) oven for 10–12 minutes, rotating the pans halfway through baking. Cookies should be golden at the edges and slightly soft in the centers — they will firm up as they cool.

Cool and finish

Remove baking sheet from the oven and let cookies cool on the sheet for 3–5 minutes, then transfer them to a wire rack to cool completely. If using, sprinkle a small pinch (about 1/16–1/8 tsp) of coarse sea salt on each warm cookie to enhance flavor.

Storage

Store cooled cookies in an airtight container at room temperature for up to 4 days, or freeze baked cookies in a freezer-safe container for up to 2 months. Alternatively, freeze unbaked dough portions on a tray, then transfer to a zip-top bag; bake from frozen, adding 1–2 minutes to baking time.