Graphene oxide (GO) nanosheets and onion-like carbon (OLC) nanoparticles were synthesized from natural graphite powder and candle soot, respectively, and characterized by transmission electron microscopy and Raman spectroscopy. The lubricating performances of GO and OLC as lubricant additives in water were comparatively evaluated using a ball-on-disc tribometer. The effects of sand blasting of a steel disc on its morphology and tribological property were evaluated. The results show that the two nanomaterials, GO and OLC, when used as lubricant additives in water effectively reduce the friction and wear of the sliding discs, which is independent of the disc surface treatment. On applying heavy loads, it is observed that GO exhibits superior friction-reducing and anti-wear abilities compared to those of OLC—a trace amount of GO can achieve a lubricating ability equivalent to that of an abundant amount of OLC. Furthermore, it is observed that sand blasting cannot improve the wear resistance of the treated steel disc, even though the hardness of the disc increased after the treatment. The possible anti-wear and friction-reducing mechanisms of the GO and OLC as lubricant additives in water are discussed based on results for the wear surfaces obtained by scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy