In a large bowl, stir together the coconut milk, maple syrup, and mashed banana. Add the oats and chia seeds, and stir to combine evenly.
Using 1 1/2 cup or 2 cup jars, place about 1/2 cup of the oat mixture at the bottom of each jar. Top with berries, then add another 1/2 cup of the oat mixture, then more berries. Cover the jars and refrigerate overnight.
In the morning, the blueberry overnight oats can be served cold, or you can heat them briefly on the stovetop or in the microwave. Garnish with lemon zest, slivered almonds, or shredded coconut.