Dependence of the photoluminescence (PL) energy (E-PL) in polar ZnCdO/ZnO quantum wells (QWs) on hydrostatic and uniaxial pressures are investigated as regards the Cd concentration of the structures and their physical dimension. The calculations are carried out within the framework of the effective mass and envelope function approximations. The built-in electric field is incorporated into the calculations using a piezoelectric polarization expression up to a nonlinear regime. It is found that the nonlinearity in piezoelectric polarization, under the effect of the hydrostatic pressure, leads to a large built-in electric field up to 5 MV/cm in consequence of the high lattice mismatch existing in the structures. The resulting quantum confined Stark effect causes a decreasing dE(PL)/dp from 23.6 meV/GPa down to even negative values, such as -6.46 meV/GP, for the QWs up to 50 angstrom in width. A linear relationship between E-PL at ambient pressure E-PLO and dE(PL)/dp is obtained in both cases of the hydrostatic and uniaxial pressure applications. It is seen that the relationship is universal in view of the Cd concentration.