An investigation on the effect of oxygen enriched air on the diesel engine combustion characteristics is carried out under part load and full load conditions. The present investigation focuses on the combustion characteristics such as heat release, the delay period and the combustion duration of a single cylinder, four-stroke, direct injection diesel engine when the intake air is enriched with 23%, 25% and 27% of oxygen by volume. The experimental results have revealed that increasing oxygen content in air leads to faster burn rates and shorter ignition delays. There is also an increase in peak cylinder pressure and rate of pressure rise which can be optimized to control emissions and also to improve the engine performance. An improvement in thermal efficiency is observed due to improved combustion performance. The experimental outcomes exposed that when diesel engine is operated with oxygen enriched air an improvement in combustion characteristics is apparent. © School of Engineering, Taylor’s University.