In a large bowl, cream together the butter, brown sugar, egg and vanilla until smooth using an electric mixer.
In a separate small bowl, whisk the flour, baking soda, cinnamon and salt together.
Add the dry mixture into the butter/sugar mixture and with the electric mixer cream little bit more until mixed well.
Using a spatula, fold in the oats, raisins and walnuts and mix well.
Chill the dough covered in the fridge for about two hours prior to baking.
Scoop 2 tbsp of dough on a baking sheet lined with parchment paper. Space the dough about 2 inches apart.
Bake for about 10-12 minutes (start checking at the 10 minute mark since all ovens are different). The cookies are done when the edges are golden but the center looks a little undercooked.
Remove from oven and keep on baking sheet for about 5 minutes prior to transferring them to a cooling rack.
Notes
To store place in an airtight container or a ziplock bag with a small piece of bread. The bread will absorb the moisture and will keep the cookies fresh longer. Store for up to 5 days.
Cookie dough can be frozen for up to 3 months. Roll into balls prior to freezing for easy thawing.