Optical second harmonic generation (SHG) is a highly surface-sensitive probe for studying crystalline Si surfaces because the inversion symmetry is broken and electric dipole optical SHG processes forbidden in the bulk are allowed. The polarized optical SHG from a perfectly oriented Si surface is inherently anisotropic, varying periodically as the in-surface projection of the polarization vector of the incident laser is rotated about a normal to the surface. The harmonic contributions to the angular anisotropy from the surface are characteristic of the surface bonding, and are modified by misorientation, chemical termination, as well as thermal treatments. This paper reviews the results of our previously reported optical SHG studies on Si(111) wafers with misorientations of 0-degrees-5-degrees +/- 0.5-degrees in the [112BAR] direction for Si-H or Si-O terminated surfaces. Azimuthal anisotropy data are compared with an empirical model for the SHG intensity that is based on (i) the nonlinear response of anharmonic oscillators, and (ii) a phenomenological theory of azimuthal anisotropies expected for different surface orientations. This model is used as a framework for estimating "effective" resonance energies from single wavelength experiments, and in particular, for providing insights into the microscopic mechanisms that can contribute to the changes in these resonance energies with respect to different processing conditions. For example, important differences between thermally grown and plasma-oxidized interfaces are identified, and cor-relations between SHG and electrical performance of the Si-SiO2 interfaces are discussed.