This study deals with the development of controlled-ignition technology for high performance compression ignition alcohol engines. The objective of this study is to make clear the main factors that govern the auto-ignition phenomenon of alcohol spray from the physical aspects. In the mixture formation process, two kinds of factors that govern the auto-ignition phenomenon of alcohol spray have to be considered. One can be called the "internal" factor and relates to fuel properties. The other can be called the "external" factor and corresponds to the surrounding gas conditions such as pressure, temperature and oxygen concentration. This paper focuses on the effect of oxygen content of entrained gas as one of the external factors affecting auto-ignition. Spray mixture formation process up to auto-ignition of ED (Ethanol-Diethyl ether) blended fuels in a large constant volume electrical heating chamber was visualized by shadowgraph method. Results showed that oxygen content of surrounding gas strongly influenced on ignition delay. Ignition delay was decreased with increasing of oxygen content for diethyl-ether-rich ED-blended fuels. However longer ignition delay was observed for gas oil when O₂ concentration was increased up to 35%. We conclude from those results that shorter ignition delay can be obtained by increasing oxygen content of entrained gas for the fuels such that auto-ignition is dominated by mixture concentration. An optimum O₂ concentration, that induces the shortest ignition delay, exists under the examined pressure and temperature conditions of surrounding gas, regardless of fuels.