Preheat the oven to 350°F (175°C). Line a 9x13-inch pan with foil and spray with cooking spray, or grease the pan directly.
In the bowl of an electric mixer fitted with the paddle attachment, cream together the softened butter and packed brown sugar until light and combined.
Add the egg and vanilla and beat until incorporated.
Add the salt, baking soda, and cinnamon and mix briefly to combine.
Add the flour and mix until just combined, then stir in the quick cooking oats with a spatula until evenly distributed.
Fold in the 2 cups of mix-ins until evenly combined.
Press the sticky dough evenly into the prepared pan, spraying your hands with cooking spray if needed. Optionally sprinkle a few extra mix-ins on top.
Bake for about 25 minutes; the center may still be slightly jiggly. Do not overbake.
Cool completely in the pan before lifting out the foil (if used) and slicing into 24 bars.