Preheat oven to 350°F. Line two baking sheets with parchment paper.
In a stand mixer, beat butter and sugars until light and fluffy, about 3–4 minutes. Add eggs one at a time, mixing just until combined. Stir in vanilla.
In a separate bowl, whisk flour, baking soda, and salt. Gradually add to the butter mixture, mixing just until combined.
Fold in oats, chocolate chips, peanut butter chips, and walnuts.
Scoop dough into 2-ounce portions and refrigerate for at least 30 minutes.
Place dough balls 3 inches apart and bake 13–15 minutes, until golden around the edges. Cool on baking sheet 10 minutes before transferring.
Notes
Use a 2-ounce cookie scoop for evenly sized cookies.
Do not skip chilling as it helps prevent spreading.
For softer cookies, bake closer to 13 minutes; for slightly crispier edges, bake closer to 15.
Dough can be frozen in scooped portions for up to 3 months.