Oil in the transformer is an insulation oil. The function is to be an insulation and cooling agent. Cooling with oil in the transformer can be achieved by entering the active part because it can be absorbed into the slot of the transformer. Testing needed to know the quality of the oil in the transformer includes color testing, water testing, acid testing and breakdown voltage is testing. The characteristics for transformer oil are important for perfect insulation because the success of insulation oil depends on composition and the age of the transformer oil. This requirement value is needed for the transformer testing in high voltage. Transformer oil requires the quality of PLN Standard, adopted IEC Standard (IEC Publication 296 and 422). From this research, it can be seen there are transformer oil insulation that can be classified as good or bad based on the testing. The suggestions to the company are to change the transformer oil which has poor insulation qualities because it will lead to poor performance of the transformer. Regular control is vital.