In a large mixing bowl, beat butter until creamy.
Add brown sugar and sugar, beating until light and fluffy.
Beat in eggs, one at a time, mixing well after each addition.
Beat in vanilla.
In a large bowl, whisk together oats, flour, cinnamon, baking soda, and salt.
Add the dry ingredients to the mixing bowl, and beat until just combined.
Fold in pecans.
Cover and chill in the refrigerator for at least one hour. If you are chilling for several hours or overnight, remove from fridge to thaw for about 15 minutes before baking.
Line baking sheets with parchment paper or silicone mats.
Drop by heaping tablespoonful about 3 inches apart.
Bake at 350 degrees for 12-14 minutes, or until edges are golden, and centers are only just set.
Remove from heat, and allow to cool on baking sheets for about 3 minutes, or until tepid to touch.
Remove to wire racks to cool completely.