This study aimed to investigate the influence of oxygen flow rate on magnetic and high-frequency properties of as-deposited FeHfO thin films, which were fabricated by DC reactive magnetron sputtering in an Ar + O2 atmosphere. These films exhibited mixed phases of α-Fe nanograins and amorphous HfO2 phase, showing nanocrystalline structure and soft magnetic properties. The dispersion character of FeHfO films changed from relaxation-type to resonance-type when the oxygen flow rate varied from 1.0 to 1.2 sccm. Furthermore, the Fe58.8Hf13.1O28.1 film with the optimum O2 flow rate of 1.2 sccm exhibited high saturation magnetization of 10.6 kG, a high resistivity of 258 µΩ cm, a real component of permeability (µ′) of 250 up to 1 GHz and a high ferromagnetic resonance frequency of 2.5 GHz. It is expected that this film should be promising for practical applications as a high-frequency ferromagnetic material.