The purpose of this study was to determine the effect of the partial pressure of O2 in the intake charge of an I.D.I. diesel engine on the various operating parameters and the exhaust emissions. The oxygen content in the intake was varied between 21% and 40% by volume. Engine performance and emissions were evaluated at constant engine speed and injection timing while fueling was varied. The research revealed that enriching the intake air with oxygen led to a large decrease in ignition delay and reduced combustion noise. The fuel economy, the power output and the exhaust temperature remained almost constant. HC and CO emissions decreased and smoke levels dropped substantially, while NOX emissions increased pro-rata with the O2 added.Theoretical analysis using the Zel’dovich kinetics showed that nitric oxide emissions are sensitive to “mean NO-formation temperature” and the combustion duration associated with this temperature, and, to a lesser extent, the atomic oxygen concentration. The theoretical analysis revealed close correlation between calculated and measured NO emissions.