Put all of the ingredients into a large mixing bowl.
Give it all a really good mix to combine everything well.
Transfer to airtight containers and refrigerate overnight (or for at least 4 hours).
Notes
In the morning, give the oats a quick stir. If they seem too thick, add a small splash of milk to loosen them.
If you are making several jars at once, stir each one really well before refrigerating. Chia seeds can clump if they are not mixed properly, so it helps to stir once, leave the jar for 5 minutes, then stir again.
Do not overdo the chia seeds. They help thicken the oats, but too much can make the texture overly dense.
If you find frozen berries very tart, add a little extra maple syrup in the morning after tasting.