Hi~

Some soup is cooked with oil in it, which might make it more delicious. I am wondering the cook first puts water in the pot, and then put oil in the water, or adversely.

Adversely. Oil is used to sauté meat and vegetables before water is added. If the meat or veggies are not sautéd first, they tend to taste boiled.

But if I first fry the oil, and then put water in it, would it become like fire-cracker?

Cripes how much oil are you talking about here? If its just a splash to fry in then no obviously not as the oil gets soaked up by what ever your cooking off.

It is actually a kind of seasoning soup, for I just want to put some vegetables and meat in the soup to make them tasty. No body want to eat the soup. I think one third of soup would be filled with oil.

Find a recipe for the soup. Some soups are supposed to be oily, like peanut butter soup, but people should enjoy it when it's done.

Most soups that I know of don't need extensive amounts of oil, even potato soup only needs 2/3 c of butter, veggie soup- no additional oil, so I really don't know beside sauteing veggies, or meat why you would add excess oil? Meat and veggies have enough flavoring, so why would you add excess fat? Some butter or a splash of oil seems to me is quite enough if your making a bean type soup the pork back would give off enough flavor as well as grease. Oil basically is fat, you just don't need excessive amounts of it for flavor or health.