Preheat the oven to 350 degrees Line two large baking sheet with Silpat baking mats or parchment paper. Set aside.
In a medium bowl, stir together the oats, flour, baking soda, and salt. Set aside.
Using a stand mixer or hand held mixer, beat the butter, peanut butter, and sugars together until creamy and smooth, on medium speed. Scrape down the sides of the bowl with a spatula, if necessary. Beat in the egg and vanilla extract. Mix until well combined.
Add in the dry ingredients and mix on low until just until the combined. Don't over mix. Gently stir in the peanut butter chips, Reese's Pieces, and chopped peanut butter cups.
Scoop the cookies into round balls, about 2 tablespoons of dough per cookie, and place on the prepared baking sheets, leaving 2 inches in between cookies. Bake for 10-12 minutes, or until the cookies are lightly browned around the edges, but still soft in the middle.
Remove cookies from oven and sprinkle the cookies with sea salt. You can also gently press additional chips, candies, and peanut butter cups on top of the cookies to make them look extra pretty. Let the cookies cool on the baking sheet for about 5 minutes or until set. When set, transfer cookies to a wire rack and cool completely.
Notes
You can gently press additional Reese's Pieces candies and chopped peanut butter cups on top of the cookies right after they come out of the oven to make them extra pretty. That is what I do:)