Preheat your oven to 350 degrees F. Line a baking sheet with parchment paper.
In a medium bowl, whisk together flour, baking soda and salt. Add oats and chopped pecans and stir well to combine.
Place light brown sugar and granulated sugar in a separate large bowl. Add melted and cooled butter and stir until smooth. Add egg and vanilla and stir to combine.
Slowly add dry mixture to wet mixture and stir to combine.
Drop by heaping teaspoons (1 ½ teaspoons of dough per cookie) onto prepared baking sheet approximately 3 inches apart.
Bake for 7 to 8 minutes (watching closely to avoid over-baking), or until golden brown and crispy around the edges.
Allow cookies to cool on baking sheet for 5 minutes, then transfer to a wire rack to completely cool.