A detailed computational analysis of the oxidizer preflow during startup of a storable propellant upper-stage rocket engine is presented. This low-pressure flow regime is controlled by various two-phase phenomena, particularly flash atomization and flash evaporation of superheated liquid oxidizer, leading to a significant temperature dropinthe combustion chamber.Toaccount for these phenomena, physical models are extendedtolow- pressure conditions and implemented into the framework of an iterative Euler-Lagrange method. A new semi- implicit concept of the spray source term linearization is employed, improving the robustness of the numerical solution procedure by accounting for intense coupling of vapor flow and spray. Following the analysis of the three- dimensional two-phase flowfield and spray deposit distribution in the combustion chamber and nozzle extension, the influence of liquid injection temperature and initial droplet size distribution is investigated by detailed parametric studies. The results indicate a particular importance of the spray-wall interaction and secondary-droplet breakup for the coarser sprays and a bimodal deposit distribution on the chamber walls. Computational results agree well with experimental data and are used to derive transfer functions describing global preflow dynamics on a system level.