Preheat oven at 350-degree Fahrenheit and lightly grease an 8-inch cake pan. (You may also place a round parchment paper after greasing the cake pan if desired. Make sure there is no hanging parchment paper from the cake pan).
In a mixing bowl combine oat flour, almond flour, baking powder, baking soda, salt and orange zest. Mix and keep it aside.
In a blender, combine 4 oranges, maple syrup, vanilla extract and orange extract and blend it smooth. Keep it aside.
Now, with the help of sieve/strainer strain the blended orange juice directly into the flour mixture. You may use your spoon to squeeze out the juice as much as possible and then discard the leftover pulp. Add soaked flaxmeal to the flour mixture and whip it into a thick pouring consistency batter.
In the cake pan, pour the batter and tap it gently for even spreading. Place it in the middle rack of the oven and bake it for 27-30 minutes or until the tester comes out clean. Turn it off and take it out. Let it cool down on a flat surface.
In the meanwhile, in a blender combine cashew nuts, oat milk, maple syrup, orange zest, orange extract and juice 1 orange in it directly. Blend it smooth until creamy and silky. (About 3-4 minutes). Keep it aside.
Now, turn over the cake on a flat surface (gently remove the parchment paper if using it) and spread the frosting on its top with a knife or a flat spatula. Garnish with more orange zest and sliced oranges and serve.