Power transformers are key components applied in transmission and distribution systems. Their power rating and life-cycle characteristics are strongly dependent on thermal aspects. Although today’s transformers are highly efficient, their cooling systems still have to discharge non-negligible amounts of heat dissipated in their windings. The higher the evolving temperature levels at a given loading rate are settling, the faster the insulation materials will age at that loading rate. Moreover, a thermal failure results not from the overall condition of the insulation but from the most degenerated part of the insulation caused by the highest local temperatures. Therefore, a profound knowledge about the temperature distribution inside a transformer is crucial for an appropriate assessment of the component’s power rating. Consequently, an efficient thermal design offers the opportunity to significantly cut costs in the production process and to improve the thermal performance. This contribution describes an approach to calculate the detailed temperature distribution inside an oil cooled winding of a power transformer by application of computational fluid dynamics (CFD) and comparison the results with measurements in order to validation. The winding model is integrated in a laboratory setup, which allows the exact control of the boundary conditions, e.g. the oil flow rate and inlet temperature. The presented results give a deep insight into the oil flow and temperature behavior of OD cooled windings enabling the designer to optimize the cooling of power transformer windings. Keywords: Power Transformer, Windings, Thermal Modelling, CFD, Hot Spot Temperature.