High entropy oxides are commonly associated with high configurational entropy (Δ S_conf≥ 1.61R) corresponding to five equimolar cations occupying a crystallographic sublattice. However, recent experimental observations indicate that medium-entropy compositions may also exhibit entropy-stabilized rock-salt phases, raising an important question regarding the minimum entropy required for phase stabilization. In this work, we employ a first-principles thermodynamic framework to investigate the stability of rock-salt oxides containing two to five principal cations components analogous to (Ni_(0.8)Cu_(0.2))O, (Ni_(0.6)Cu_(0.2)Zn_(0.2))O, (Ni_(0.4)Cu_(0.2)Zn_(0.2)Co_(0.2))O, (Ni_(0.2)Cu_(0.2)Zn_(0.2)Co_(0.2)Mg_(0.2))O. Density functional theory, MCSQS-based structural modeling, and finite-temperature Gibbs free-energy analysis are combined to quantify the roles of enthalpy mixing (Δ Hₘᵢₓ), configurational (Δ S_conf), vibrational (Δ S_vib), electronic and magnetic (Δ S_mag) contributions towards (Δ S_elec) entropy change in governing phase stability. The results show that Δ S_conf alone is not a universal descriptor of phase stability. While the two-cation system is enthalpy-stabilized but three-, four- and five-cation systems become thermodynamically stable at high-temperature due to entropy-driven reduction of the Gibbs free energy. These findings demonstrate that single-phase rock-salt oxides are not restricted to the conventional high-entropy limit and that medium-entropy compositions can also be stabilized under suitable thermodynamic conditions.