The influence of the partial pressure of oxygen(PO2) on the microstructural and magnetic properties of sputtered Co–Fe–Hf–Ofilms has been studied. It is shown that the films prepared underPO2 = 6–11.5% with large saturation magnetization,4πMs∼18–21 kG, large hard-axisanisotropy field, HkH∼30–84 Oe, andhigh electrical resistivity, ρ∼1400–3600 µΩ cm, are excellent candidate materials for high-frequency applications ofmicromagnetic devices such as magnetic thin film inductors, transformers andthin film flux gate sensors. In particular, the good soft magnetic properties ofCo19.35Fe53.28Hf7.92O19.35 nanocomposite films of 50–437 nm thickness, in addition to their high electrical resistivity,make them ideal for use in micromagnetic devices with an opening bandwidth of severalgigahertz. This is attributed to the formation of a peculiar nanostructure in these samples.A strong magnetic phase separation appears to occur as the film thickness increases over437 nm, which, in turn, modifies the high-frequency magnetic behaviour of theCo19.35Fe53.28Hf7.92O19.35 film.