The paper deals with the results of numerical and experimental studies of gas-turbine unit oil air cooler. The experimental study was performed under operating conditions of main pipeline gas-compressor station. The numerical simulation was carried out by means of finite element method in STAR-CCM+ program. The Hungarian heat exchanger cooling oil of the gas-compressor unit based on gas-turbine unit GTN-16 was chosen as the research object. The aim of the study was to evaluate oil cooler heat transfer rate before and after its air circuit modernization. The modernization was to install a leveling apparatus that distributes the air flow uniformly in front of the tube bank. Design and optimization of the leveling apparatus geometry were made by means of the verified finite element model. The paper describes cooler design and operation principle. The design shortcomings that were the reason for the study are presented. The measuring scheme and field test methods are described. The author gives the examples of the air flow velocity field in front of the finning before and after modernization as results of the study. The paper states the enhancement of the oil cooler heat flux after the air flow velocity field leveling.