Preheat oven to 375ยบ F. Line a baking sheet with parchment paper.
In a medium sauce pan, heat butter and brown sugar over medium heat, stirring, until mixture is melted and smooth. Remove from heat. Stir in oats, pecans, flour, and salt, stirring until combined. Stir in egg and vanilla until combined.
Spoon cookie batter by the teaspoon onto prepared baking sheets, leaving about 2 inches between each cookie (they will spread quite a bit).
Bake for 5 -7 minutes, until golden brown, being careful not to burn.
Allow to cool on the cookie sheet until firm enough to transfer, then remove onto cooling rack and cool completely.