In a large mixing bowl, cream together butter, brown sugar, and sugar for 4 minutes or until light and fluffy.
Add peanut butter, egg, and vanilla and beat for 1 minute longer.
Fold in oats, flour, baking soda, and salt. Stir just until combined.
Fold in chocolate chips and ¾ cup of M&M's, reserving ¼ cup for after baking.
Cover the dough and chill for 30 minutes.
Once the dough has chilled, preheat oven to 365 degrees.
Scoop the dough onto a light colored baking sheet, spacing at least two inches apart.
Bake for 9-10 minutes.
Remove from the oven and immediately lightly press the remaining ¼ cup of M&M's into the tops of the cookies.
Let cool for 5-10 minutes before removing.
