Ionic liquids are promising electrolytes for electrochemical gas sensors that have unique physicochemical properties such as negligible vapor pressure and high thermal stability. The modification of ionic liquid (IL) by combining metal oxide with reduced graphene oxide (rGO) is an effective method to improve its gas-sensing properties. In this study, the mesoporous structure of NiCoO/rGO is synthesized by simple one-step method, and the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF]) is mixed with it to form the composite material NiCoO/rGO/[BMIM][PF]. Electrochemical test results indicate that the three electrolytes exhibit response current and long-term stability in the oxygen environment. The oxygen sensor based on NiCoO/rGO/[BMIM][PF] significantly improves the response current and working stability of pure ionic liquid. The sensitivity of the sensor is 0.1087 μA/[%O], and the linear regression coefficient of the reduction peak current calibration curve is 0.9995. After continuous cyclic voltammetry, the reduction peak current remains at 90% of the initial current value. The interaction of IL and NiCoO/rGO significantly enhances electrochemical oxygen sensing performance.