Preheat oven to 350 degrees. Prep your two baking sheets by lining with parchment paper or silicone mats.
Add eggs and sugars to a large bowl of a stand mixer. Fit mixer with paddle attachment. Turn mixer to low to combine eggs and sugar together until smooth.
Add salt, vanilla, peanut butter, and unsalted butter to sugar mixture. Mix on mediu until smooth.
Add M&M's, chocolate chips, baking soda, and oats. Very carefully, turn mixer to low. You don't want the oats to spray everywhere. You can also do this by hand with a spatula if you're scared of overflow.
Scoop cookies using a tablespoon cookie scoop. Place on two prepped baking sheets, about 1.5 inches apart. They don't spread very much so flatten the tops of the cookies using two fingers and lightly press down. You won't be able to fit all cookie dough on two sheets so you'll have to do another batch once the first batch is done baking.
Bake cookies for 8-10 minutes, until the edges start to brown. Do not let them completely brown. When they cook for too long, they will no longer be chewy. These cookies taste so much better when they're chewy. Let cool for 5 minutes before transferring to cooling racks.
Keep stored in a storage container for up to 7 days. I also like to display them in a cake stand with a lid for easy access. Enjoy!