Preheat the oven to 325.
Beat the butter, shortening, water, and white sugar until smooth, about 2 minutes. Add egg and vanilla and mix until just combined.
Add baking powder, salt, flour, oats, and Oreos and mix until just combined. The dough should not be very sticky and should be quite thick.
Use an extra large cookie scoop to scoop out the dough (each dough ball should weigh about 100g or 3.4 oz). Flatten each dough ball with a cup or with your hand. Put up to six dough balls on a normal sized cookie sheet and cook in the oven for 12 minutes.
When you take the cookies out they will look doughy, but do not fear! They are done! When you take the cookies out of the oven, put them directly into the fridge or freezer.
While the cookies are chilling, make the frosting. Whip the butter, powdered sugar and vanilla extract for 2 minutes. Slowly add water until desired consistency is reached (I usually add about 2 tbsp). Let the mixture whip on high for 5 minutes until the frosting is smooth and fluffy.
Once the cookies have been in the fridge at least 15 minutes, frost each cookie and put them into the fridge for at least an hour to let the frosting set up! If you are in a hurry you can put them into the freezer, just make sure you let them thaw for a few minutes before eating.
When the cookies are chilled and the frosting has set, grab them out of the fridge and eat them up!
