This study examined the effects of the oxygen partial pressure on the properties of tin oxide (SnOx) thin films deposited by radio frequency magnetron sputtering using a SnO target. The properties of the samples were characterized by Hall Effect measurements, dynamic-secondary ion mass spectrometry, X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and atomic force microscopy. All the samples exhibited dominant Sn2+ XPS peaks, indicating that SnO with p-type conductivity was the main composition regardless of the oxygen partial pressure. The samples deposited with an oxygen partial pressure of 12% showed the best p-type characteristics, which included a maximum hole mobility of 1.94 cm²/Vs, carrier concentration of 3.83×1017/cm³, Sn2+ peak area percentage of 91.34%, Sn4+ peak area percentage of 2.35%, and Sn0 peak area percentage of 6.31%. As the oxygen partial pressure was increased to more than 12%, the Sn2+ peak area percentage decreased while the Sn4+ peak area percentage increased. This was attributed to the reduction of the SnO phase and the growth of the SnO₂ phase in the samples due to the incorporation of more oxygen. These results are expected to contribute to the development of p-type SnO-based TFTs with good performance.