Measure ½ cup of old-fashioned, rolled oats inside each lidded jar. Alternatively, you can place everything in a large lidded container and measure out individual portions. Add any dry "add-ins" at this time. These are optional.
Pour ¾ cup of water inside each jar or a total of 3 cups into the large container. Add any fruit at this time, chopped into small pieces, if using.
With a spoon, mix the oat and water mixture. Place the lid on the jar or seal the container.
Place the jars in the refrigerator for 4 hours or overnight.
In the morning, enjoy cold or warm up in the microwave for 30 seconds to 1 minute, stir, and serve.