Place four jars on a flat surface. Fill each jar with ½ cup oats, ½ teaspoon ground flaxseed, ¾ cup milk, and ½ teaspoon vanilla (if using). Stir to combine and top each jar with ½ cup of frozen fruit.
Refrigerate:
Refrigerate for 8 hours or overnight.
In the morning, stir to combine. Warm up the oats in the microwave if desired, or enjoy them cold.
Notes
If using large pieces of frozen fruit, like peaches, strawberries, or pineapple, run the fruit under warm water for about a minute (or leave them out on the counter for 5 minutes) to thaw out slightly and use a large knife to chop the fruit into smaller, bite-sized pieces.