Tried this last night literally because of this thread. I normally make hot oats in the morning, so thought this would be a fun change. Here's what I did:
1 and 1/3 of a cup of rolled oats
1 cup 1% milk
1 overly ripened banana
roughly 1 teaspoon of cinnamon
I mashed up the banana a bunch, then threw in the oats, milk, and cinnamon. I mixed it all up inside a plastic tupperware, snapped the lid on it, and left it in the fridge overnight (~8 hours). In the morning, I opened it, mixed it up briefly, then topped it with some yogurt. It was pretty good even without the yogurt. I'll make it again sometime. Also, I have some frozen fruit I may try to mix in the night before next time to see how that tastes.
You could also use a not overly ripened banana, but I happened to have one handy and it was a bit too soft for me to enjoy eating on its own, but was perfect for this purpose.