Preheat the oven to 350°F (175°C) and grease a 9x9-inch baking dish.
In a large bowl, combine the rolled oats, baking powder, ground cinnamon, and salt.
In another bowl, whisk together the almond milk, maple syrup, eggs, vanilla extract, and Greek yogurt until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. If using, fold in the chopped nuts and dried fruit.
In a small bowl, mix together the brown sugar, ground cinnamon, and melted coconut oil or butter to create the cinnamon swirl.
Pour half of the oatmeal mixture into the prepared baking dish. Drizzle half of the cinnamon swirl mixture over it, then add the remaining oatmeal mixture on top. Finish with the rest of the cinnamon swirl, swirling it gently with a knife to create a marbled effect.
Bake for 30-35 minutes, or until the top is golden brown and the center is set. Let it cool for 5-10 minutes before slicing.