This work studies the heavy oil gasification experimentally and numerically. The experimental investigations use the semi-industrial test facility HP POX (High Pressure Partial Oxidation gasifier), operated at the Institute of Energy Process Engineering and Chemical Engineering, TU Bergakademie Freiberg. The HP-POX facility with a capacity of 5 MW operates under industrial conditions and features comprehensive instrumentation. The numerical model considers state-of-the-art model approaches and a detailed determination of the droplet size distribution, applying breakup models to the liquid heavy oil phase. The optical system of the HP-POX allows the heavy oil injection to be observed; these observations are then used to evaluate the droplet size distribution. The numerical model approach is validated against two lab-scale experiments from the literature, which also provide data on high pressure and temperature conditions. Validation A is the pressurized entrained-flow reactor of the Commonwealth Scientific and Industrial Research Organisation applying coal gasification at 20 bar. Validation B is the research gasifier for liquid fuel, of the Central Research Institute of Electric Power Industry that gasifies Orimusion (R) at 19 bar. The numerical model shows good agreement for both experiments, and for the heavy oil gasification experiment in the HP-POX plant. The validated model is used to study the impact of two different burner concepts on gasification characteristics. Burner 1 has a central fuel injection and an annular gasification agent injection, while Burner 2 has a central gasification agent injection and an annular fuel injection. The results show that Burner 1 achieves a higher syngas yield (H-2 + CO) but with higher soot production in comparison to the simulation results for Burner 2.