Rise in Transformer Oil Temperature

What do you think the cause might be?

Then if you can decide that, you can work on the solution.

Have you done any research on the specific transformer you have or is this homework?

If you really have a transformer, call an electrician.

http://en.wikipedia.org/wiki/Transformer_oil_testing

http://www.electrical4u.com/transformer-oil-and-winding-temperature-rise-test/

http://en.wikipedia.org/wiki/Transformer_oil

The cause is energizing the transformer, the solution is to keep it de-energized. Seriously, the oil in a transformer is there to provide insulation and cooling. Anything electrical that has current flowing through it heats up as a result of I²R losses, more current more heat. The oil in a transformer moves that heat from the windings and core to the outside world without ever touching it except through a heat exchanger.

Do some basic research into "transformer cooling/heat rise" to learn more.

Is this a home work or what?