<p>Equimolar (CoCuMgNiZn) entropy stabilized oxide (ESO) is the first ever reported composition in this emerging class of materials. Its phase formation, stability and microstructural evolution have been re-investigated by subjecting it to different thermal treatments. The microstructure and phase of the ESO have been characterized by X-ray diffraction (XRD) and electron microscopy. The ESO forms a cubic rock salt structure globally. The shouldering, splitting and non-ideal intensity distribution, observed in the XRD pattern, may be attributed to the local structural modulation with subtle variation in chemistry in the ESO, instead of attributing it to the diffraction by Cu-K_<i>α</i> and Co-K_<i>α</i> doublets. Electron diffraction patterns consistently exhibit splitting, arcing and geometric shape evolution of spots. Structural modulation in sintered and quenched ESO leads to the formation of mutually rotated tweeds, which grow further to form domains with sluggish kinetics upon aging. Structurally correlated phases also grow with definite orientation relationship within the rock salt phase with coherent interfaces. The ESO minimizes its volumetric strain, lattice strain and interface strain by formation of tweeds, inter-grown domains and structurally correlated phases with coherent/semi-coherent interfaces in association with subtle variation of chemistry. The stability of the ESO is not solely determined by the maximization of configurational entropy due to the presence of five different cations in the cation sublattice, rather it is a trade-off between the configurational entropy and the strain energy that stabilizes the phase and the microstructure in the time-temperature space.</p>