Nowadays, a stack of heavily doped polysilicon (poly-Si) and tunnel oxide (SiO_x) is widely employed to improve the passivation performance in n-type tunnel oxide passivated contact (TOPCon) silicon solar cells. In this case, it is critical to develop an in-line advanced fabrication process capable of producing high-quality tunnel SiO_x. Herein, an in-line ozone-gas oxidation (OGO) process to prepare the tunnel SiO_x is proposed to be applied in n-type TOPCon solar cell fabrication, which has obtained better performance compared with previously reported in-line plasma-assisted N₂O oxidation (PANO) process. In order to explore the underlying mechanism, the electrical properties of the OGO and PANO tunnel SiO_x are analyzed by deep-level transient spectroscopy technology. Notably, continuous interface states in the band gap are detected for OGO tunnel SiO_x, with the interface state densities (D_it) of 1.2 × 10¹²–3.6 × 10¹² cm⁻² eV⁻¹ distributed in E_v + (0.15–0.40) eV, which is significantly lower than PANO tunnel SiO_x. Furthermore, X-ray photoelectron spectroscopy analysis indicate that the percentage of SiO₂ (Si⁴⁺) in OGO tunnel SiO_x is higher than which in PANO tunnel SiO_x. Therefore, we ascribe the lower D_it to the good inhibitory effects on the formation of low-valent silicon oxides during the OGO process. In a nutshell, OGO tunnel SiO_x has a great potential to be applied in n-type TOPCon silicon solar cell, which may be available for global photovoltaics industry.