Place the 1/4 cup roasted pistachios in a food processor or small blender and pulse until they become small crumbs—almost like pistachio powder; avoid overprocessing into butter. Set aside.
Bring 2 cups water to a boil in a medium saucepan over medium-high heat.
Add 1 cup old-fashioned rolled oats, 1/2 teaspoon salt, and 1/2 teaspoon ground cinnamon to the boiling water, then reduce heat to a simmer.
Stir in the ground pistachio crumbs and cook, stirring often, until the oats are thick and creamy, about 10 to 15 minutes.
Remove the pan from the heat and stir in 1 teaspoon vanilla extract, 1/3 cup coconut milk, and 2 tablespoons maple syrup until combined.
Spoon the oatmeal into bowls and top each serving with chopped pistachios, dark chocolate chips or chunks, flaked coconut, and hemp hearts; drizzle extra coconut milk and maple syrup if desired.