This is for whole/large flake oats, in case you're referring to something else?

I cook these almost daily in the winter, and occasionally otherwise.
It's pretty much 2:1 water with a dash of salt.
You may boil the water first, but I combine them first (makes it creamier).
Anyway, bring to a soft boil, stirring occasionally to prevent sticking to bottom of pot, turn heat off (leave the pot on the stove though), cover, and let sit for ~10 minutes.
After you've made it a few times you should be able to eyeball it, adding water or oats as required.

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.

Edit: removed answer that was unrelated to the OPs question

If you are doing savoury, like for dinner, replace the water with broth. If doing sweet for breakfast milk and sugar are great. 3 cups is about right, but you need to cook for longer until it is all absorbed. Simmer for 45 mins or so, stirring occasionally to prevent the bottom from burning.

2 cups is probably insufficient. Boil turn off and wait hours is also not the preffered method, less time, more low heat. Taste ocasionally until you get the desired texture, then consider adjusting broth / water / milk levels.

thanks everyone for the suggestions. second run: 1 cup groats (thanks for the term), 1 cup water, splash of olive oil, pinch of salt. cover, bring to boil, switch to lowest setting, let simmer. got a glass lid, so monitoring progress.

20 min - still water left. 30 min - still some water visible. 40 min - none visible, occasional buble pops through. turn off heat, leave covered for 10 mins. sadly, there's some water at the bottom, was hoping everything will get absorbed.

third run will be one setting above lowest for the simmer part thus hopefully shortening it to 30 mins, and then leaving it covered for 15 minutes, hopefully that will do the trick.

The ratio is probably similar to steel cut oats, but you might need a longer cooking time. A couple of hours without heating is probably excessive and only increases the chance of food poisoning.

Try a 40 min simmer with the lid on.