Oil-immersed transformers are primarily categorized by their cooling medium, heat dissipation structure, and protection method. The cooling method is the most crucial criterion, typically represented by letter codes: Oil-immersed natural cooling (ONAN) relies on natural oil circulation and air convection to dissipate heat, offering a simple structure and requiring minimal maintenance; Oil-immersed air cooling (ONAF) utilizes a fan to enhance heat dissipation, significantly increasing capacity; and Forced oil-circulation air cooling (OFAF) or forced oil-water cooling (OFWF) utilizes an oil pump to accelerate oil flow and supplements this with strong air or water cooling, suitable for extra-large transformers. Furthermore, based on the casing structure, transformers can be categorized as either standard oil tank type or fully sealed (with expander). The latter effectively prevents air and moisture ingress, slowing oil aging. Based on the protection configuration, transformers can be further categorized as standard or protected with a gas relay and pressure relief valve, enhancing early warning and protection against internal transformer faults.