Chronological account on the development of oxygen sensors for use in liquid sodium has been made and reviewed. Factors like chemical compatibility with sodium, electrolytic domain boundary, electrical conductivity, thermomechanical property, etc. of yttria doped thoria (YDT) and yttria / calcia stabilized zirconia (YSZ or CSZ) were compared. YDT was found to be superior to zirconia based electrolytes for the intended application in sodium. Analyses of different designs evidenced improved life of sensors using YDT in thimble form.