Yes, of course. I have done this both ways, and I can tell you that the chia seeds help the mixture thicken and attain a creamier texture, almost like it would be if you were to cook the oatmeal. Without the chia seeds, the oats absorb the milk, but you’ll also sometimes have some milk unabsorbed, with kind of a watery texture. If this is unimportant to you, the chia seeds are not necessary.