Thermal oils are widely used as heat transfer fluids in medium temperature applications. Addition of small amounts of nanoparticles in such fluids can significantly improve their thermophysical properties. This paper presents experimental investigation of an oil-based nanofluids prepared by dispersing different concentrations (0.25 wt�1.0 wt) of copper oxide nanoparticles in Therminol-55 oil using two-step method. Shear mixing and ultrasonication were used for uniform distribution and de-agglomeration of nanoparticles to enhance the stability of the suspensions. The effect of nanoparticles concentrations on thermophysical properties of the nanofluids was analysed by measuring thermal conductivity, dynamic viscosity, effective density and specific heat capacity at different temperatures (25 °C�130 °C). Thermal conductivity exhibited increasing trend with rising temperature and increase in nanoparticles loading. A significant decrease in dynamic viscosity and effective density against increasing temperature makes it suitable for medium temperature applications. Nano-oils with improved thermal properties are expected to increase the efficiency of concentrating solar thermal collectors. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim