Preheat the oven to 350ºF (175 °C). Line two cookie sheets with parchment.
In a bowl, whisk together the flour, oats, baking soda, and salt.
In a large bowl with an electric mixer (I like this one from Braun on Amazon), cream together the butter and the sugars for several minutes.
Add the egg and the vanilla. Mix them in well, scraping down the sides of the bowl with a spatula as needed.
Add the dry ingredients and stir them in. When the dry ingredients are almost incorporated, add the chopped chocolate and the peanuts. Fold them in.
Scoop the dough, rolling into even balls and place them on the cookie sheets, you may only fit 8 cookies per sheet depending on the size of the sheet. Press the cookies lightly to flatten them out a little with the palm of your hand,
Bake the cookies in the preheated oven for 12 minutes—less for a gooey-style softer oatmeal cookie (like 10 minutes), more for a crispy edged chewy oatmeal cookies (like 14 or even 16 minutes).
Let cool slightly before transferring the baked cookies to a cooling rack to cool completely.
Notes
I like to use the electric hand mixer from Braun (available on Amazon)
For the milk chocolate, I used Cacao Barry Alunga 41% milk chocolate.