Preheat oven to 350 degrees.
Line a 9x9 inch square pan with parchment paper or foil.
In a bowl, whisk together flour, baking soda, salt, and oats; set aside.
In a large bowl, cream together butter, peanut butter, and sugars until light and smooth, about 3 minutes.
Add egg and vanilla extract and beat for an additional 2 minutes.
With the standing mixer or beaters running on low, gradually add the flour mixture, mixing only until just combined. Stir in M&M’s.
Evenly press dough into the prepared pan. Bake bars for 18-20 minutes or until edges are golden and a toothpick is inserted into the center and comes out clean. Cool completely.
Place the chocolate chips in a small microwave safe bowl. Microwave on 50% power for 45 second intervals until the chocolate chips are melted; whisk until smooth.
Drizzle melted chocolate over bars and garnish with M&M’s. Let chocolate drizzle harden for about 30 minutes at room temperature or refrigerate for 15 minutes before slicing.
Cut bars into 9 large squares or 12 mini squares.