Preheat the oven to 350°F. Line a large baking sheet with parchment paper or a silicone baking mat and set aside.
In a medium bowl, whisk together the oats, flour, baking soda and salt. Set aside.
In a large bowl, beat together the butter and sugars until creamy. Then add the peanut butter and beat an additional minute. Add in the eggs and vanilla and mix until well combined. Slowly mix in the oat mixture and mix just until combined.
Stir in the M&M's and chocolate chips, reserving some for the tops if desired. Add the pretzels if desired.
Use a large cookie scoop to scoop dough onto prepared baking sheet, about 2 inches apart. Lightly press down on the dough balls with the palm of your hand. Add additional M&M's and/or chocolate chips to the tops.
Bake for 10-11 minutes, or until the edges are lightly browned. Allow cookies to sit on the baking sheet for 3-5 minutes and then transfer to a cooling rack. Repeat with the remaining dough.
Store in an airtight container at room temperature for up to 3 days or freeze up to 3 months.Note: To make mega monster cookies, form the dough into about 16 large round balls (1/3 cup of dough). Then bake an additional 2-3 minutes. Recipe from The Two Peas and Their Pod Cookbook