In a bowl or mason jar, combine the rolled oats, unsweetened almond milk, full-fat Greek yogurt, maple syrup, vanilla extract, and chia seeds. Stir vigorously for about 1-2 minutes until everything is well combined and the mixture has a consistent texture. The chia seeds will begin to absorb liquid and help thicken the oats as they sit. I like to use a bowl rather than a jar because it's easier to mix everything evenly and prevents the oats from settling into a dense layer at the bottom.

Gently fold in the fresh or frozen blueberries, reserving a small handful for topping. If using frozen blueberries, there's no need to thaw them first—they'll soften as the oats sit and release their flavor into the mixture. Cover the bowl and refrigerate for at least 6 hours, or preferably overnight. The oats will absorb the liquid, the chia seeds will expand, and the texture will transform into a creamy, pudding-like consistency.

Remove the overnight oats from the refrigerator and give them a good stir. The mixture should be thick and creamy, but if it's too dense or you prefer a looser consistency, add a splash more almond milk (1-2 tablespoons at a time) and stir until you reach your desired texture. I find that a thicker consistency works best for eating with a spoon, but some people prefer it more pourable. Transfer to a serving bowl or eat directly from the container, then top with the reserved blueberries.