Abstract: Forced oil directed air-forced (ODAF) transformers, owing to their complex internal oil-duct structures and strong dependence on active cooling components, are highly sensitive to the operating conditions of the cooling system. Once cooling system malfunctions occur—such as fan failures, degradation of heat-exchange performance, or abnormal operation of oil pumps—rapid increases in oil and winding temperatures may occur, posing a serious threat to the safe operation of the transformer. Focusing on the technical characteristics of ODAF transformers and the operational requirements of their cooling systems, this paper investigates the cooling system operating principles, control strategies, operating modes, fault types, and corresponding mitigation measures.