Combine flour, oats, cinnamon, baking soda and salt in a medium bowl and whisk to blend evenly.
Combine butter, brown sugar and granulated sugar in a large bowl and mix it together using a wide rubber spatula until smooth, slightly pale and fluffy-looking. It will take 1-2 minutes. Mix in molasses until combined. Add egg and vanilla and mix well until evenly incorporated and smooth.
Add in the flour mixture and mix until just combined with a few streaks of flour still remaining, then add raisins, walnuts and dark chocolate. Fold it all in until evenly distributed.
Use a 1.25-oz cookie scoop to portion dough and place them at least 3 inches apart on prepared baking sheets. Bake for 10-13 minutes until very golden and still slightly soft in the middle. The cookies may look slightly under baked in the center but the edges will be deeply browned.
Let cookies cool on the baking sheets over a wire rack for 5 minutes to set, then transfer cookies individually to the cooling rack to cool completely.