Use a 2:1 ratio of oatmeal to lentils. Double that amount and add that much water (for example if you used ½ cup total of lentils and oats, add 1 cup water). You can also try using milk or milk alternative.
In a pot, bring the oats, lentils and liquid to boil. Turn down to a simmer and cook until all water is absorbed.
Add your favourite toppings.