In a large mixing bowl, beat the butter, sugars, eggs, and vanilla until well blended.
Add the peanut butter and mix thoroughly.
Add the oatmeal. Sprinkle the baking soda over the oatmeal, and then mix to fully incorporate both in the dough.
Add the chocolate chips and M&Ms and mix to combine. Note: If you prefer the M&Ms to remain more intact, see note below.
Using two teaspoons, drop rounded teaspoons of the dough onto ungreased cookie sheets, pushing them off the first spoon using the second spoon. I fit 12 cookies per sheet, 3 across and 4 down, leaving about 2 inches between cookies. Cookies flatten and spread out a bit.
Bake for 10-12 minutes until brown and crispy.
Let the cookies cool ON THE PAN for 3 minutes before removing them to a cooling rack to fully cool. Then store in an airtight container.
Notes
Note 1: Use a silicone baking mat on top of your cookie sheet and clean up will be a breeze.Note 2: Wait to add the M&Ms until you have baked the cookies for 5 minutes if you want the M&Ms to remain more intact. Just take the pan out of the oven, press a few M&Ms into the top of each cookie, and return the sheet to the oven to finish baking for the remaining time.