This study aims to analyze rapeseed oil’s effectiveness as a coolant of a high-voltage direct current (HVDC) converter transformer while comparing it with mineral oil. We employed a numerical model, assisted by Computational Fluid Dynamics (CFD) tools, to simulate the convective heat transfer inside a HVDC converter transformer, which is an oil-directed air forced (ODAF) type. The model seeks to obtain the temperature and velocity fields at the critical points of the fluid domain. The simulation domain reproduces a 2D cross-sectional model in the center of the structure. The results show that rapeseed oil requires more time to gain heat from the core and windings due to its lower thermal conductivity. However, the average temperature of rapeseed and mineral oils will not significantly differ once the channel temperature reaches a stable value. It indicates that the rapeseed oil can effectively replace mineral oil under the same heating conditions. Also, a slower flow of the rapeseed oil allows the fluid to smoothly enter the cooling channel. Therefore, the rapeseed oil cools these channels by pushing the fluid in them and absorbs heat from the sources earlier than mineral oil. The improvement in cooling indicates that rapeseed oil is better than mineral oil as a coolant and electrical insulating fluid for power transformers.