Where the transformer oil is circulated in the transformer and how it's cooling the transformer?

Depends on the size of the transformer. Are you are talking about a large power transformer, say 2.5MW or above? If so then the following is usually true. The core/windings of the transformer are immersed in the oil. When the transformer is energised and carrying load the winding heats up. This in turn heats the oil. The tank of the transformer is usually cool due to the air surrounding it. The hot oil moves away from the core to the cooler tank sides. This starts the oil circulating. i.e. as it moves to the side it cools down and then returns to the centre and gets heated up again. As the transformer load increases so does the winding temperature. This means that the oil has to dissipate more heat. Eventually the capacity of the oil to dissipate the heat quickly enough will be reached. If this temperature is reached before the transformer is fully loaded that damage may occur Increasing the size of the tank is one solution However, this could mean a very large piece of equipment. The normal practice is to install a cooling bank or set of "cooling fins". They are also sometimes known as radiators. Generally they are separate from the transformer and connected to it by a flow and return pipe. The flow to pipe is high up near the top of the transformer and the return is near the base. A pump is then used to circulate the oil through them. The pumps are temperature controlled. They only come on when the oil has reached a certain temperature. If this cooling is not sufficient then fans are added. These fans are positioned under the cooling banks. When they are switched one they force air over the cooling banks. This additional air flow takes the heat way faster thus cooling the oil quicker. The pumps then return it to the main tank where it is heated by the core/ windings again.