Combine rolled oats, almond milk, Greek yogurt, honey, and vanilla extract in a medium mixing bowl. Stir until all ingredients are evenly distributed and no dry oat clusters remain visible.

Add ground cinnamon and sea salt to your mixture, stirring thoroughly to ensure the spices distribute evenly. Let the mixture sit for 2 minutes, then give it another quick stir before moving forward with layering.

Divide the oat base equally between two mason jars or airtight containers. Pour half into each container, pressing gently to settle the mixture against the bottom of each vessel.

Layer half the fresh blueberries, strawberries, and raspberries into each jar on top of the oat base. Sprinkle the sliced banana pieces and ground flaxseed over the berries to create your cozy summer berry overnight oats recipe foundation.

Top each jar with remaining almonds for crunch and texture contrast. Cover tightly and refrigerate for at least 8 hours, ideally overnight, allowing flavors to meld and oats to absorb the liquid completely.

In the morning, stir your overnight oats thoroughly before eating, as separation is completely normal and expected. Add an extra splash of almond milk if you prefer a thinner consistency than what developed overnight.