Actually we use water for preventing the fire to stop but it is also said that water contains oxygen which helps in combution so why is it used so???????

Water does contain certain percentage of dissolved oxygen, However, this percentage is not sufficient enough to promote the spreading of fire. Moreover, water is an excellent coolant. It brings down the temperature of the combustible substance below its ignition temperature, which helps in extinguishing the fire. It also cuts off the supply of air, thereby further controlling the fire.