In the bowl of a Stand Mixer fitted with the paddle attachment, cream the butter and sugar until creamy. Add in the eggs, one at a time mixing just until incorporated. Add in the vanilla and blend.
In a separate bowl mix together the flour, baking soda, salt, and cinnamon. With the mixer on low add in flour mixture until blended. Add the oats and mix until just combined. Remove the bowl from the mixer, scrape off the paddle.
Fold in the apples and 1 cup of butterscotch chips using a sturdy spoon or spatula. Place 1/4 cup into each whoopie pie cavity or on the cookie sheet, 2” apart. Bake for 9 to 11 minutes. Remove the pan from the oven and place the pan on a cooling rack allowing the cookies to cool in the pan for a minute.
Using a spatula, carefully remove the cookies from the pan and place them on a cooling rack. While the cookies are cooling, place the remaining butterscotch chips and vegetable oil in a microwave safe bowl and microwave in 15-20 second increments until smooth and creamy. Drizzle the butterscotch over top using a piping bag, plastic baggie with the tip cut off, or just a spoon. Allow to cool completely.