Overnight oats must be refrigerated because there is protein, fats, carbs, oxygen, and moisture in the jar. Not refrigerating could put overnight oats in what is called the temperature danger zone (TDZ), a 41°F to 135°F range where bacteria can grow, with 70°F to 104°F being range where they grow the fastest. Overnight oats are safe properly refrigerated for up to 5 days. If you are taking a serving with you for a grab and go breakfast, they will be safe for 2 hours unrefrigerated. Otherwise, keep them on ice packs in a cooler. Safe refrigerator temperatures are below 40° but not so cold that food freezes. I recommend setting between 34°F to 38°F, just a few degrees short of the max safe level.