Application of a theory of nuclear symmetry-based reaction restrictions to the O-2 + O --> O-3 reaction provides a potential explanation for the symmetry-induced isotopic enrichment observed for laboratory and atmospherically produced O-3. Within this theory, the rate of formation of O-3 from collisions of O and isotopically homonuclear O-2 depends on whether the O-2 molecule is in an f (allowed) or an e (restricted) parity label state. The restriction can be relaxed by various potential energy surface coupling terms, and the assumption that approximately 78 percent of the restricted O-2(e) levels produce O-3 with the same efficiency as the allowed O-2(f) levels can account for laboratory-observed isotopic fractionation. In particular, the theory explains the special enhanced formation of the completely asymmetric isotopomer (OOO)-O-16-O-17-O-18.