AbstractOxocarbenium ions are key reactive intermediates in organic chemistry. To generate a series of structure‐reactivity‐stereoselectivity principles for these species, we herein investigated the bimolecular electrophilic substitution reactions (S_E2′) between allyltrimethylsilane and a series of archetypal six‐membered ring oxocarbenium ions using a combined density functional theory (DFT) and coupled‐cluster theory approach. These reactions preferentially proceed following a reaction path where the oxocarbenium ion transforms from a half chair (³H₄ or ⁴H₃) to a chair conformation. The introduction of alkoxy substituents on six‐membered ring oxocarbenium ions, dramatically influences the conformational preference of the canonical ³H₄ and ⁴H₃ conformers, and thereby the stereochemical outcome of the S_E2′ reaction. In general, we find that the stereoselectivity in the reactions correlates to the “intrinsic preference” of the cations, as dictated by their shape. However, for the C5‐CH₂OMe substituent, steric factors override the “intrinsic preference”, showing a more selective reaction than expected based on the shape of the ion. Our S_E2′ energetics correlate well with experimentally observed stereoselectivity, and the use of the activation strain model has enabled us to quantify important interactions and structural features that occur in the transition state of the reactions to precisely understand the relative energy barriers of the diastereotopic addition reactions. The fundamental mechanistic insight provided in this study will aid in understanding the reactivity of more complex glycosyl cations featuring multiple substituents and will facilitate our general understanding of glycosylation reactions.