The voltage-controlled magnetic anisotropy (VCMA) effect is a key to realising high-speed, ultralow-power consumption spintronic devices. The fcc-Co-(111)-based stack is a promising candidate for the achievement of large VCMA coefficients. However, only a few studies on the fcc-Co-(111)-based stack have been reported and the VCMA effect has not been well understood. Previously, we observed a significant increase in the voltage-controlled coercivity (VCC) in the Pt/Ru/Co/CoO/TiOxstructure upon post-annealing. However, the mechanism underlying this enhancement remains unclear. This study performs multiprobe analyses on this structure before and after post-annealing and discusses the origin of the VCMA effect at the Co/oxide interface. X-ray magnetic circular dichroism measurement revealed an increase in the orbital magnetic moment owing to post-annealing, accompanied by a significant increase in VCC. We speculate that the diffusion of Pt atoms into the vicinity of Co/oxide interface enhances the interfacial orbital magnetic moment and the VCMA at the interface. These results provide a guideline for designing structures to obtain a large VCMA effect in fcc-Co-(111)-based stacks.