Oil-immersed transformers are power transformation devices that utilize oil cooling to reduce transformer temperature. Unlike dry-type transformers, the core and windings of oil-immersed transformers are housed within a welded steel tank filled with insulating oil.

During operation, heat generated by the windings and core is first transferred to the insulating oil, which then acts as the cooling medium.

Based on capacity, they can be further classified into immersion-type naturally cooled transformers and immersion-type forced air-cooled transformers.