In this article, we focus on optical and magneto-optical properties of Au@CoO core-shell nanoparticles synthesized by a solution-phase seeded growth route under ambient atmosphere, in which the pyrolysis of Co(acac)2 in oleylamine is conducted in the presence of preformed Au nanoparticles. Optical absorption exhibits a broad peak that can be ascribed to localized surface plasmon resonance (LSPR) of the Au counterpart, but its position is red-shifted as compared to that of pure Au nanoparticles, meaning that the Au cores are coated by a dielectric material (=CoO) with high permittivity. Nanoscopic analysis based on X-ray photoelectron spectroscopy (XPS) only weakly implies the slight existence of zero-valent Co metal. In contrast, magnetic circular dichroism (MCD) shows a very intense characteristic response that is completely different from those of both Au and CoO counterparts but close to metallic Co, demonstrating that the MCD response is dominated by a very small amount of metallic Co within the core-shell nanostructures. Hence MCD analysis can be a very efficient way with a great potential for definitely identifying the presence of slight ferromagnetic components in non-magnetic nanostructures.