Preheat oven to 350 degrees.
In a large bowl, beat together butter and sugars until light and fluffy. Beat in egg and vanilla.
In a separate bowl, combine flour, oats, flaxseeds, baking soda, cinnamon and salt. Stir into butter mixture until it forms soft dough.
Drop cookies by the tablespoon onto baking sheet lined with parchment or silpat, about 2 inches apart.
Bake for 10-12 minutes. Let cool on rack for 2 minutes before moving them to cooling racks.
Store in an airtight container at room temperature for up to 5 days, or freeze for up to 1 month.