This paper aims at studying the effect of oxygen enriched combustion on performance, emission and combustion characteristics of a diesel engine using waste cooking oil (WCO) derived from palm oil as fuel. A single cylinder water-cooled, direct injection diesel engine was used. The intake system of the engine was modified to accommodate excess oxygen in the incoming air. Base data was generated using diesel as fuel. Subsequently experiments were repeated with WCO for different oxygen concentrations such as 21% (WCO+21%O2), 23% (WCO+23%O2), 24% (WCO+24%O2) and 25% (WCO+25%O2) by volume. Engine performance, emission and combustion parameters were obtained at different power outputs and analyzed.Results showed reduced brake thermal efficiency, higher smoke, hydrocarbon and carbon monoxide emissions with WCO+21%O2 as compared to diesel at all power outputs. The brake thermal efficiency was found as 26.2% with WCO+21%O2 where as it was 30.5% with diesel at the rated power output of 3.7 kW. The smoke emission was noted as 75% and 40% respectively with WCO+21%O2 and diesel respectively. There is an improvement in brake thermal efficiency with the oxygen enrichment technique. The brake thermal efficiency was noted as 27.2%, 28.4% and 27.8% with WCO+23%O2, WCO+24%O2, and WCO+25%O2 respectively at 3.7 kW. The smoke emission was considerably reduced with WCO in oxygen enrichment operation. The lowest smoke (50%) was found with WCO+25%O2 at the maximum power output. Hydrocarbon and carbon monoxide emissions were considerably reduced with all concentrations of oxygen in the intake air. However, NO emission increased to high levels with increase in oxygen percentage in the air.At peak power output, the ignition delay was observed as 13°CA for WCO+21%O2. However, ignition delay was reduced considerably with oxygen enrichment. The minimum delay was noted for WCO+25%O2 as 9°CA where as it was 10°CA with diesel. Cylinder peak pressure of WCO was measured as higher with oxygen enrichment as compared to atmospheric oxygen concentration at all power outputs. Reduction in combustion duration and improvement in heat release rates were observed with oxygen enrichment as compared to 21% oxygen at all power outputs. It was concluded that oxygen enrichment technique can be adopted for reducing smoke, hydrocarbon and carbon monoxide emissions of a diesel engine fuelled with WCO as fuel. The optimum concentration of 24% of oxygen can be preferred in order to avoid over heating of the combustion chamber parts and without having very high values of NO emissions at peak power output.