Mix oatmeal and water, then soak for 5 minutes.
Put the oatmeal and water in a pot and bring to a boil.
Drain oil from tuna using a strainer.
Once the porridge boils, add tuna, salt, and pepper. Stir and cook for 5 more minutes.
Slice green onion to your preference and add if desired.
Serve in a bowl to complete.