One part grain, two parts water, salt to taste (or soy sauce for tastier taste).
Bring to boil, then let it simmer on the smallest flame your stove can do. Keep the lid covered. When you stop hearing boiling noise, it’s ready. Takes around 30-40 minutes, if it cooks longer you’re probably using too much water.
Basically, prepare it like buckwheat. No need to pre-soak it, oat grain is soft enough that you can chew it unboiled. Washing before boiling is up to personal preferences, it’s gonna boil so it’s sterile by default.