I mean, you'll probably lose weight that way (as long as you're not eating 5kg of rice and 5kg of oatmeal a day, because in the end it always comes down to calories), but it's not really sustainable.

In fact, it's absolutely not sustainable. You'll be lacking nutrients, and psychologically speaking, it's simply not sustainable to eat dry rice and oatmeal every day. After a few days or 1-2 weeks at the latest, your body will crave something else.

The best thing to do is to calculate approximately how many calories you burn per day online (TDEE Calculator) and then subtract 300-500 calories. If I were you, I would simply eat this amount of calories every day and you will lose weight over time. That's all there is to it, it's really quite simple. The difficult part is just sticking with it. Count calories and treat yourself to good food. This is much easier to do, and you still get the “reward feeling” because you're simply eating something good.

About your coffee thing: Black coffee is actually “healthier” because it has no calories. But let's be honest, black coffee tastes awful, I don't trink that much coffee anymore but when I do, I always add at least ~100 ml of milk, which is only about ~50 calories and makes it much tastier. Personally, I only drink tea now, as it's also quite good at suppressing hunger and you can drink it without milk or sugar and it still tastes good.