Place oatmeal, milk, molasses and oil in a medium bowl, stir, and set aside.
Heat oven to 350F. Grease and flour, or use baking spray, to prepare 2 8-inch round cake pans.
In medium bowl, whisk together flour, baking powder, baking soda, cinnamon and salt. Set aside
In bowl of stand mixer, fitted with the paddle attachment, beat butter on medium speed until smooth. Add in granulated sugar, brown sugar, and vanilla bean paste and beat until light and fluffy, 3-5 minutes. Scrape down the sides and while mixer is on low, add eggs one at a time, scraping down sides as necessary.
While mixer is on low, alternate flour mixture and oat mixture in 5 additions, beginning and ending with the flour mixture. Scrape side of mixing bowl to make sure batter is completely combined, then evenly divide between two cake pans.
Bake for 30-35 minutes, or until a cake tester or wooden skewer inserted into the middle of the cake comes out clean. Let cool in pans for 10 minutes, then place on wire rack to cool completely.
Vanilla Bean Frosting
Cut butter into small slices and place into bowl of stand mixer with paddle attachment. Add salt. Whip butter for 5-7 minutes until light and airy. Scrape down sides of bowl as necessary.
Add 3 cups powdered sugar and mix on low to combine. Add in vanilla bean paste and 1 tablespoon heavy cream, then combine.
Add 3 cups powdered sugar and mix on low to combine. Add 1 tablespoon heavy cream. Scrape down sides as necessary. Add in remaining 2 cups powdered sugar and remainging tablespoon heavy cream and and mix to combine. Beat on medium-high for 5 minutes to whip. Scrape sides of bowl and re-whip if needed.
Place frosting, marshmallow creme and powdered sugar in small bowl. Using hand mixer, combine completely
Level cake tops. Slice each cake in half to create 4 even cake layers. Place a small amount of frosting on cake board to secure cake or place cake on serving plate. Using a piping bag fitted with a large round tip, pipe around the top of the cake to create a dam for the marshmallow filling. Fill with 1/3 of the marshmallow filling. Repeat with layers 2 and 3.
Place last layer on top upside-down and apply a crumb coat* over cake. Place in freezer for 15-20 minutes.
Frost top and sides and smooth with cake scraper/comb. Place in freezer for 15-20 minutes to chill. Place extra into piping bag with Wilton 2D star tip. Pipe 12 swirls on top
Cut 3 oatmeal cream pies into quarters and place one piece on each swirl. Crumble remaining oatmeal cream pies and place around base of cake. Serve or refrigerate then bring to room temperature before serving.
* Crumb coat is a thin layer of frosting applied after stacking the cake, so the crumbs do not show when the final layer of frosting is applied.