The site selectivities and stereoselectivities of C-H oxidations of substituted cyclohexanes and trans-decalins by dimethyldioxirane (DMDO) were investigated computationally with quantum mechanical density functional theory (DFT). The multiconfiguration CASPT2 method was employed on model systems to establish the preferred mechanism and transition state geometry. The reaction pathway involving a rebound step is established to account for the retention of stereochemistry. The oxidation of sclareolide with dioxirane reagents is reported, including the oxidation by the in situ generated tBu-TFDO, a new dioxirane that better discriminates between C-H bonds on the basis of steric effects. The release of 1,3-diaxial strain in the transition state contributes to the site selectivity and enhanced equatorial C-H bond reactivity for tertiary C-H bonds, a result of the lowering of distortion energy. In addition to this strain release factor, steric and inductive effects contribute to the rates of C-H oxidation by dioxiranes.