Yes, overeating is the only reason for obesity. You consume energy and breathe out carbon just by being alive. If you don't replace it with food, you will lose weight, that's physics.

The hard problem is what makes you overeat. It can be a psychological problem, some kind of unbalance with the feeling of satiety, food that is too energy-rich for the amount of nutrients it contains,...

All diets only have one goal: make your effective energy intake lower than what you consume. It can be by simply making you eat less, or by making you eat food that triggers the feeling of satiety before you have eaten too much (ex: lots of energy-poor fiber).

I think there are exceptions, some people seem to store ridiculous amount of water in their body. They are not that fat, but their body is bloated by all that water. I think it is the case for extreme obesity.