In order to optimize decarburization in the RH degassing process, it is necessary to develop sensors capable of continuous and direct measurements of an oxygen concentration. However, such measurements are only possible for a few seconds with conventional sensors. Here, the effects that hinder long-time measurements by a single Mo/MoO2–type zirconia oxygen sensor were investigated and an approach to extend sensor lifetime was developed by the direct current (DC) voltage application. Reference electrodes containing molybdenum and molybdenum oxide were constructed in a single-closure tube of zirconia solid electrolyte. The EMF between the reference electrode (negative) and the sample electrode (positive) inserted in molten iron decreased from +300 mV owing to the formation of a metallic molybdenum layer on the inner surface of the zirconia tube. This layer was attributed to reduction of molybdenum oxide gas owing to oxygen diffusion from the inside to the outside of the tube. Because the oxygen diffusion coefficient in molybdenum is much lower than the oxygen ion diffusion coefficient in the zirconia, the layer disrupted oxygen transfer and reduced the EMF. Additionally, a DC voltage of 2.0 V was applied to the reference electrode and EMF recovered over 20 minutes. The molybdenum layer disappeared and the reference electrode was re-exposed to the electrolyte. Thus, the applied DC voltage enabled long-time measurements of oxygen content with a single Mo/MoO2–type zirconia oxygen sensor.