The thermal behaviour of several ONAN (Oil Natural Air Natural) distribution transformers has been numerically modelled. A simplified differential model has been developed with the aim of reducing the computational cost that would require a model with the complete geometrical description. This model has been capable of reproducing the expected oil flow and the thermal distribution inside the transformer. The influence of turbulence modelling in the obtained results has been evaluated and the model has been verified in terms of discretization errors. The thermal boundary conditions have been thoroughly analysed searching for the most appropriate expressions for this particular case instead of using inadequate mean values obtained from bibliography. The devised model has been validated by comparing the numerical results with the experimental ones obtained for different transformers and power losses. This mathematical tool can be used to study the natural convection of the oil inside the transformers and allows the manufacturers to optimise their designs from a thermal point of view. (C) 2010 Elsevier Ltd. All rights reserved.