We present a comprehensive model of ferroelectricity in oxygen perovskite crystals that account quantitatively for both displacive like and order-disorder like properties. The model assumes that in the paraelectric phase, the system has a soft mode, interacting with spontaneous disordered off-center displacements. The resulting theory is shown to quantitatively explain the properties of two representative perovskites: KNbO3 and PbTiO3. We have further extended this theory to explain the properties of mixed incipient ferroelectrics with small concentrations of ferroelectricity inducing ions such as KTa1-xNbxO3. The model quantitatively reproduces the basic experimental data in the quantum regime: the temperature dependence of the inverse dielectric function, the impurity and temperature dependence of the renormalized soft mode vibrational frequency and the impurity concentration dependence of the transition temperature Tc. In particular we show that, the critical concentration for Tc=0 is determined by the ion tunneling frequency, the bare soft mode parameters and the pseudospin - phonon coupling constant.