What is the technical difference between oil cooled transformer air cooled transformer?

The basic difference between the air cooled transformer and oil immersed transormer is: If the heat generated in the transformer winding is capable of being swapped by the sarrounding ambient air without causing any problem to the performance of the transformer then its normally reffered as Naturally Air Cooled transformer or simply air cooled transformer. However when the capacity of the transformer increases, the heat generated in the winding of the transformer is more and the ambient air normally is inadequate to cool the windings and maintain the performance of the transformer. Its because of this reason the transformer windings are submerged in an oil tank. It has been observed that the hydrogen ions present in oil help to maintain the temprature of the winding of the transformer and thus overall performance of transformer.