Angle-dependent x-ray magnetic circular dichroism at the Co L2.3 edges has been utilized to systematically study Co80Pt20 : WO3 perpendicular magnetic recording thin films, in which the magnetocrystalline anisotropy significantly drops as the oxide volume fraction increases. The microscopic origin of this phenomenon in the studied films can be mainly attributed to an increase in orbital moment normal to the grain–oxide interface, with increasing oxide volume fraction, which arises from a more pronounced effect of symmetry breaking at the grain–oxide interface in smaller grains.