Combine oats and 2 cups of buttermilk in a medium mixing bowl and set aside for 10 minutes.
In another bowl combine the flour, brown sugar, molasses, baking soda, baking powder, and salt. Add remaining buttermilk and eggs to the oat mixture and stir until blended. Fold the dry ingredients into the liquid mixture until well combined. Stir in the Grape-Nuts. (Thin batter with a splash of buttermilk if needed.)
Preheat a large cast iron skillet or griddle on medium-high heat and add about a tablespoon of canola oil. When the oil is hot and shimmery, add 1/4 cup portions of batter to the pan, pressing it out with the bottom of the measuring cup to help it spread out a bit. Lower the heat to medium and cook until the bottoms are golden brown, 1 to 2 minutes, then flip and continue cooking until done, 2 to 2 1/2 minutes more. Re-heat the griddle before cooking additional batches of pancakes.