Air injection into oil reservoirs has been widely applied in oilfields for improved oil recovery, while the safety of the process is of great concern due to the explosion of the mixtures of natural gas (mainly methane) and injected air at high pressure and high temperature. Oxygen-reduced air flooding (ORAF) has been proposed to eliminate the explosion risks. The combustion and flammability characteristics of methane-air mixtures at elevated pressure and temperature are also important for natural gas engines and air compressors. In this study, an experimental setup was used to measure the flammability limits and explosion energy of methane in oxygen-reduced air (ORA) at pressures up to 15 MPa and 373 K and compared with methane air mixtures. The experimental results show that the flammability limits of methane in ORA can be significantly reduced or narrowed, which can reduce explosion risks. At high pressures, the low flammability limit of methane in ORA is reduced, and its upper flammability limit is increased, which follows a similar pattern with methane-air mixtures. The explosion energy of methane in ORA can be also greatly reduced due to the lack of oxygen. Finally, the explosion risks in applying the ORAF technique for improved oil recovery have been analyzed.