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.