Preheat oven to 350 degrees.
Beat butter, margarine and both sugars at medium speed with an electric mixer until creamy. Add eggs, one at a time, beating just until yellow disappears after each addition. Mix in vanilla, baking soda, baking powder and salt until well incorporated. Add flour one cup at a time until combined.
Stir in oats, chocolate chips and pecans. Drop by tablespoonfuls (or use smallest cookie scoop) onto greased baking sheets. Bake for 12-14 minutes. Cool on baking sheets 3 minutes; remove to wire racks to cool completely.
Dark brown sugar is a MUST for these - it really makes the blondie flavor come out.