It has been recognized that an ultrathin insulator interface (oxide) between a semiconductor and a metal contact enhances the performance of an MOS solar cell over the corresponding MS structure. Also it is now recognized that a transition region of 10-20 Å exists at the Si-SiO2interface. Because the oxide layer thickness in MOS Si solar cells is comparable to the transition region, the nonstoichiometry and pinholes associated with the ultrathin oxide layer will affect the performance of the MOS solar cell. In the present paper the nonstoichiometry of the ultrathin oxide layer is treated by varying the parameters associated with the Si-SiOxinterface and the pinholes in the oxide have been treated by associating a different barrier height to the pinhole regions. The resulting theoretical model is shown to explain the experimental variation of open-circuit voltage with the oxide layer thickness.