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High aspect ratio etching is crucial in semiconductor manufacturing, and its improvement relies on the stability and controllability of the plasma. In this study, we combine Langmuir probe diagnostics with a global model to investigate how O2 addition affects plasma characteristics in CF4/Ar radio frequency (RF) inductively coupled discharges. The key operational parameters include the CF4 mixing ratio (binary: 30%–90%, ternary: 10%–40%), working pressure (1–8 Pa), and RF power (200–500 W). The results show that in CF4/Ar discharge, the electron density decreases, whereas the effective electron temperature increases as the CF4 content is raised from 30% to 90%. When O2 is added to the CF4/Ar discharge with the Ar fraction fixed at 50%, the effective electron temperature also increases with the CF4 content in the range 10%–40%, while the electron density remains nearly constant. Analysis of the electron energy probability functions reveals that O2 addition results in low-energy electrons remaining constant with varying CF4 content. Global model calculations further indicate that O2 diversifies the plasma chemical composition and enhances the formation of F and F2 through reactions with CFx. The resulting high density of F2, combined with its large dissociative attachment cross section, makes dissociative attachment to F2 the dominant electron-loss pathway. Furthermore, as the pressure increases from 1 to 8 Pa, the electron density in CF4/Ar discharge exhibits a non-monotonic behavior characterized by an initial increase followed by a decrease. In contrast, it decreases monotonically in O2/CF4/Ar discharge. This work systematically reveals the influence of O2 addition on fluorocarbon plasma characteristics, offering guidance for optimizing plasma etching processes with improved stability and controllability.