Sacrificial thermal oxidation of standard SIMOX is currently the main route to form ultra thin film SIMOX structures. During the oxidation process self interstitials are injected into the silicon overlayer and these point defects can lead to the growth of secondary defects, in particular oxidation induced stacking faults (OISF). The formation of these OISF is influenced by the presence of stacking fault tetrahedra (SFT) in the silicon overlayer. In this paper we investigate the formation of OISF using transmission electron microscopy (TEM) and a recently developed chemical defect etchant. We propose a model which describes the evolution of OISF from the existing SFT.