Numerical simulations of diesel reacting sprays in a simulated engine environment were carried out to study the influence of oxygen concentration on the ignition delay and lift off length dynamics. A recently developed n-dodecane kinetic mechanism developed by Samimi et al (2016) was used with the direct integration detailed chemistry approach and classical Kelvin Helmholtz Rayleigh Taylor models to describe the reacting liquid spray. The conditions studied were in the range from of 16-21% oxygen concentrations at surrounding oxidizer conditions of 850K, and 60bar. An axial 90 micron single-hole orifice diesel injection configuration was used with injection durations of 0.71, and 1.2ms while employing the Adaptive Mesh Refinement technique and grid-convergent methods. The simulations are able to provide a time-history of chemical species for 60 species including formadelhyde [CH2O] intermediates and hydroxide [OH] radicals to facilitate development of auto-ignition and lift off length numerical diagnostics. The results demonstrate that the kinetic model is able to capture the transient combustion behavior accurately with oxygen concentration sensitivities. Furthermore the simulations demonstrate that auto-ignition and lift off are independent of injection duration. The predicted behaviors are also in good agreement with measurements from the Constant Pressure Flow (CPF) facility at ARL. Fuel injection and combustion processes are highly relevant to liquid fuel chemical propulsion applications for ground and aerial propulsion applications of interest to the Army.