Maybe it's a regional thing, but "breakfast cereal" is exclusive of "oatmeal" where I'm from (despite oats being a cereal grain). Breakfast cereal is crunchy and served cold, neither of which applies to oatmeal (even overnight oats are cold but not crunchy).
Overnight oats / bircher muesli is served cold and requires no cooking. The time aspect is to give the oats time to absorb the liquid - cold oats swimming in freshly poured milk isn't the most appetising way to eat oats.
Of course you can "speed up" the process by blending them into a smoothie.
You can (overnight oats), but it's still not crunchy.
If you toast the oats first to make them crunchy, it's no longer the dish called "oatmeal" (which is a porridge) but now it's a kind of granola, which can be used as a breakfast cereal.
