Prepare the pumpkin mixture. In a large mixing bowl, whisk together the pumpkin purée, Greek yogurt, oat milk, maple syrup, and pumpkin pie spice until smooth and creamy.
Add the oats and chia seeds. Stir in the rolled oats and chia seeds, making sure everything is evenly incorporated.
Refrigerate overnight. Cover the bowl or divide the mixture into individual airtight containers. Refrigerate for at least 4–6 hours, preferably overnight, allowing the oats and chia seeds to absorb the liquid and develop a creamy consistency.
Prepare the apple topping. Dice the apples into small, bite-sized pieces, keeping the skin on for additional fiber and polyphenols. For the freshest texture, prepare the apples just before serving.
Assemble and serve. Divide the overnight oats into five portions. Top each with diced apples and approximately 1½ tablespoons of pepitas. Sprinkle with additional cinnamon if desired.
Enjoy! Serve chilled or gently warm the oats before adding the apple and pepita toppings.