The behavior of oxygen gas participating in fining was observed in CRT (Cathode Ray Tube) glass melts doped with $Sb_2O_5;or;CeO_2$ by means of a yttria-stabilized zirconia (YSZ) electrode. The temperature dependence of the oxygen equilibrium pressure ($P_{o2}$) or the activity in both melts showed typical behavior corresponding to a theoretical redox reaction. In other words, the $P_{o2}$ value of melts with $CeO_2$ was lower than that of melts with $Sb_2O_5$ above $1250^{circ}C$. The result implies that $Sb_2O_5$, is more efficient as a fining agent compared to $CeO_2$. On the other hand, melts from a batch containing $Sb_2O_5;and;KNO_3$ showed much higher $P_{o2}$ values compared to melts without $KNO_3$ above $1350^{circ}C$. It is suggested that the addition of $KNO_3$ to CRT glass batch contributes partly to the first fining of the melts.