We present what we believe to be the first theoretical study of optical parametric oscillations in isotropic semiconductor microcavities. We have made use of the well-known possibilities of Bragg mirrors with very high reflectivities. The parametric oscillation conditions are derived in a very general and comprehensive way. Design rules are thus defined for optimum nonlinear microcavities. This theory is applied to GaAs materials (a semiconductor of the optoelectronic main stream) in which it is shown that threshold conditions far below optical damage may be obtained for reasonable values of the mirror reflectivities.