Oxygen content of Ni-based superalloy powders is higher than those of their bulk alloy counterparts due to the larger specific surface area of the former, which is detrimental to the performance of powder metallurgy (PM) and additive manufacturing (AM) superalloys. Therefore, at present, research in this field is primarily focused on understanding the mechanism of oxygen content increase of the powders and approaches of oxygen decrease. Storage and degassing treatment are typical processes of increasing and decreasing of oxygen content in superalloy powders, respectively. Studying the effects of these processes is of great significance for guiding the optimization of powder treatment processes and further improving alloy properties. The original surface state of powders with different narrow particle size ranges, as well as the effects of oxygen increasing/decreasing processes, i.e. storage and degassing, on the microstructure and mechanical properties of alloys were investigated using field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), focused ion beam (FIB), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and temperature programmed desorption with mass spectrometry (TPD-MS). The results indicate that the surface composition of the original powders with different particle sizes has no significant difference, all samples exhibit NiO/Ni(OH)₂, TiO₂, CoO, and Cr₂O₃ on their surfaces. The average thickness of the surface oxide layer for 0-15 μm fine and 150-180 μm coarse powders is 3.32 and 10.90 nm, respectively. The oxygen content of the 0-15 μm fine powders and 150-180 μm coarse powders gradually increases in ambient air environment and stabilize at about 250 × 10^-6 and 40 × 10^-6, respectively, within 3-10 d. The oxygen content of the bulk alloy consolidated from the post-storage powders (0-53 μm) increased compared to that of the alloy from pre-storage powders, and the tensile strength at room temperature, 650^oC, and 750^oC showed minor changes, but the ductility decreased and the stress rupture properties of the alloy at 650^oC, 890 MPa and 750^oC, 530 MPa decreased. During the heating process from room temperature (~25^oC) to 1000^oC, the gas desorption occurred on the 0-15 μm fine powders, with desorption peaks of CO₂, H₂O, and H₂ observed. The gas desorption mainly occurred on the powders surface in the range of 100-600^oC, and the desorption peaks are mainly located within 300-600^oC. However, the desorption peaks were not obvious during the heating of the 150-180 μm coarse powders. The oxygen content of the alloy consolidated from powders with particle size range of 0-53 μm decreased from 195 × 10^-6 in the initial state to 113 × 10^-6 after the (300^oC + 600^oC) combined degassing process. Alloys prepared from powders that underwent combined degassing exhibited higher mechanical properties, with the performance improvement mainly reflected in the ductility index of the alloy. The oxygen increase mechanism of superalloy powders mainly includes surface oxidation and surface adsorption, while the oxygen decreases mainly due to the desorption of oxygen-bearing gases on the powder surface. The temperatures of the peak position in the desorption curves of superalloy powders were selected to accurately customize the holding temperature of the degassing process. As a result, through multi-stage degassing treatment at 25^oC + 150^oC + 310^oC + 470^oC, the oxygen content of the powders (0-53 μm) stored in ambient air was further reduced to within (87-96) × 10^-6.