In a large bowl or jar, combine the rolled oats, almond milk, honey or maple syrup (if using), vanilla extract, cinnamon, and salt. Stir well to combine.
Gently fold in the sliced banana and blueberries, reserving a few for topping if desired.
If using Greek yogurt, fold it in at this stage for added creaminess.
Divide the mixture evenly into two jars or containers with lids.
Seal the jars and refrigerate overnight (or for at least 4 hours) to allow the oats to soak and soften.
In the morning, give the oats a good stir. Add a splash of milk if they are too thick. Top with reserved banana slices and blueberries before serving.