Preheat oven to 350°F. Line 2 baking sheets with parchment paper.
In a large bowl, beat softened butter, brown sugar, and granulated sugar until light and creamy, about 2 minutes.
Beat in eggs one at a time, then mix in vanilla until smooth.
In a separate bowl, whisk flour, baking soda, salt, and cinnamon.
Add dry ingredients to the wet mixture and mix on low just until a few streaks of flour remain.
Fold in oats, chocolate chips, and walnuts until evenly distributed.
Scoop dough into 2 tbsp portions and place 2 inches apart on prepared sheets.
Bake 10 to 12 minutes, until edges are golden and centers look just set.
Cool on the sheet for 5 minutes, then transfer to a wire rack to cool completely.
For thicker cookies, chill scooped dough for 30 minutes before baking.
Notes
Use old-fashioned oats for the best chewy texture. If you only have quick oats, reduce to 1.75 cups. Dough can be chilled up to 48 hours; let sit 10 minutes at room temperature before scooping. Store cookies airtight at room temperature for 3 days or freeze up to 2 months.