The influence of the partial pressure of oxygen (P_O2) on the microstructural and magnetic properties of sputtered Co–Fe–Hf–O films has been studied. It is shown that the films prepared under P_O2 = 6–11.5% with large saturation magnetization, 4πM_s~18–21 kG, large hard-axis anisotropy field, H_kH~30–84 Oe, and high electrical resistivity, ρ~1400–3600 µΩ cm, are excellent candidate materials for high-frequency applications of micromagnetic devices such as magnetic thin film inductors, transformers and thin film flux gate sensors. In particular, the good soft magnetic properties of Co_19.35Fe_53.28Hf_7.92O_19.35 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 several gigahertz. 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 over 437 nm, which, in turn, modifies the high-frequency magnetic behaviour of the Co_19.35Fe_53.28Hf_7.92O_19.35 film.