Preheat oven to 350°F. Line a 9x13” pan with foil and spray with cooking spray. (Foil is optional; I like using it for easy removal.)
Cream brown sugar and butter in the bowl of an electric mixer fitted with a paddle attachment. Add eggs and vanilla and beat until combined. Mix in salt, baking soda, and cinnamon. Add flour and mix until combined. Stir in oats. Stir in your mix-ins.
Press into prepared pan (dough is sticky, spray your hands with cooking spray to avoid it sticking to your hands). If you want, you can sprinkle a few more of your mix-ins on the top to make them look prettier (as I did with the mini M&Ms in the pictures).
Bake for about 25 minutes. (Be careful not to overcook - the center will still be jiggly when you take these out of the oven and they will finish cooking and firming up as they cool.) Cool completely before slicing into squares.