High entropy oxides (HEOs), which contain multiple elements in the same crystallographic site, are a promising platform for electrocatalysis in oxygen evolution reaction (OER). However, to date, all OER studies involving HEOs have used it in polycrystalline bulk form. In this study, the OER activity of a set of single crystalline thin films of (La0:2Pr0:2Nd0:2Sm0:2Eu0:2)NiO3, grown on NdGaO3 substrates have been investigated. The OER activity increases with the thickness of the film. X-ray absorption spectroscopy measurements find an increase in Ni d-O p covalency and a decrease in charge transfer energy with the increase in film thickness. These facilitate higher charge transfer between Ni and surface absorbates, resulting in higher OER activity. However, the epitaxial stabilization of thicker film becomes difficult due to the requirement of +3 oxidation state of Ni and the OER activity of a 75 unit cell thick film is found to be the optimal in the present study. This work demonstrates that the thickness of perovskite oxides can be used as a parameter to enhance OER activity.