Preferably something that has little to no preparation required.
Cooking is not necessary for overnight oats. I used steel cut myself, but the texture of these oats prepared this way may be chewier than you expect or are used to though, if you have always been heat-cooking them.
Just look up overnight oats and there are plenty of recipes and suggestions, some even using yogurt instead of milk. Here’s one from Martha Stewart for starters,
https://www.marthastewart.com/1524080/no-cook-overnight-oats