Oleic acid (OA) modified ZnO nanoparticles with an average size of 4.04 nm were prepared via a facile in situ one-step route. The as-prepared ZnO nanoparticles were characterized by X-ray diffraction, transmission electron microscopy, and Fourier transform infrared spectroscopy. The light-absorbing performance and thermal stability of the as-prepared ZnO nanoparticles were evaluated by ultraviolet-visible light spectrophotometry and thermogravimetric analysis, and their tribological behaviour as the lubricant additive in poly-alpha olefin (PAO) and diisooctyl sebacate (DIOS) base oils was evaluated with a four-ball machine. The surface morphology of the wear scar was observed with a scanning electron microscope and a three-dimensional optical profiler, and the element composition of the wear scar was determined by energy dispersive spectrometry. It was found that the surface-capping of ZnO nanoparticles by OA helps to greatly increase their dispersibility and thermal stability. In the meantime, OA-modified ZnO nanoparticles can effectively improve the antiwear and friction-reducing abilities as well as the load-carrying capacity of the base oils, which is due to the formation of a tribofilm in association with the tribo-sintering and/or adsorption of the additives on the rubbed steel surfaces. Furthermore, the friction-reducing and antiwear performances of the as-prepared ZnO nanoparticles as lubricating oil additives seem to be closely related to the polarity of the base oils. Namely, the OA-modified ZnO additive exhibits better tribological behaviour in apolar PAO than in polar DIOS.