Combine all overnight oats ingredients (oats, flax, chia, cinnamon, milk, Greek yogurt, applesauce, vanilla, almond butter, and sweetener) in a jar. Stir until evenly incorporated.
Add diced apples and stir again.
Cover and refrigerate oats overnight, or at least 4 hours.
When you're ready to enjoy your oats, give them a stir to see if you like the consistency or if you need to adjust with another splash or milk or two.
Add any desired toppings and enjoy!
Note: I often add a scoop of collagen peptides or half a scoop of protein powder to these overnight oats. Collagen peptides don't typically affect absorption, but most protein powders will require a little extra milk - adjust accordingly!