Line one or two baking sheets with parchment paper or silicone baking mats, set aside.
In the bowl of a stand mixer or in a large mixing bowl with a hand mixer, cream the butter and sugars together until pale, about 2 minutes.
Add the vanilla, then the egg, mix until combined.
In a medium bowl, whisk together the oats, flour, baking soda, and salt.
Add the dry ingredients to the wet ingredients, mixing just until a few streaks of flour remain. Using a spatula, fold in the trail mix by hand until evenly distributed.
Cover the bowl with plastic wrap and chill in the refrigerator for 30 minutes.
Preheat the oven to 350 degrees.
Using a standard size cookie scoop (#40 or 1.5 tablespoons), place balls of cold dough onto the prepared baking sheets. Press reserved trail mix on top, if desired.
Place in the oven and bake for 10-13 minutes, then remove and let cool on sheets for 3 minutes before transferring to a wire rack to cool completely. Store in an airtight container for up to 3 days.