Heat the oven: Set the oven to 350°F and line two large baking sheets with parchment paper.
Whisk the dry mix: In a medium bowl, whisk the flour, baking soda, cinnamon, and salt, then stir in the rolled oats.
Cream the butter and sugars: Beat the softened butter with the brown and granulated sugars for 2 to 3 minutes until pale and fluffy.
Add the eggs and vanilla: Beat in the eggs one at a time, then the vanilla, scraping the bowl between additions.
Fold in the oats and mix-ins: Stir in the dry oat mixture just until no flour streaks remain, then fold in the raisins and walnuts.
Rest the dough: Cover and rest at room temperature for 20 minutes (or chill 30 minutes) so the oats hydrate for maximum chew.
Scoop and space: Scoop 2-tablespoon mounds onto the lined sheets, 2 inches apart, and press the tops very lightly.
Bake: Bake one sheet at a time for 10 to 12 minutes, until the edges are golden and set but the centers still look slightly soft.
Cool and set: Rest on the hot baking sheet for 5 minutes, then transfer to a wire rack to cool completely.