Based on USDA's "2-hour rule" oat milk that has been opened shouldn't be left at room temperature for more than 2 hours or 1 hour if the temperature in your home is 90°F or higher. This applies to both shelf-stable and refrigerated oat milk if the carton has been opened. The only exception is shelf-stable oat milk in an unopened carton. This type of oat milk can be stored in the pantry or a cupboard until you open it.